I know this would lessen the impact of the story but I think this is something Stewie would have done. Its would have been in his character

1 whole month passed since that awful day. The day that Brian Griffin had died, and no one took that harder then Stewie. For the entirety of the month he has stayed in his room crying on his bed. Mourning the death of his best friend, and would be love of his life. Stewie loved Brian for a long time but never got to express his love for him for very long, which made him feel much worse. All he could think about was the happy future he and Brian could have led, but he could never have it. Brian is dead, and that possible future will never happen.

As Stewie arrived home from school he overheard a news report as Chris is watching TV. The news report was covering a tragic story about a 6 year old child who was in an accident. although the child is alive the accident rendered him brain dead and is being kept alive in intensive care. Then an idea sparked in Stewie's head, he just came up with a plan so ridiculous and insane it just might work. Stewie now spent another month preparing to initiate this asinine plan.

Stewie now has everything set up in his secret lad. He stepped into his time machine and traveled back in time to the day Brian died. Over the years Stewie has made some advancement to the time machine, he can now freeze moments in time. Stewie made it to the moment before Brian had died, he froze the moment before Past Stewie kissed Brian. Stewie needed to copy Brians personality from an alive brain, a dead one would be useless. He put a helmet on top of past Brians head and turned it on, the helmet scanned every inch of Brians brain. His personality, his memories, his experiences, everything. Once that was done he removed the helmet, went back to the present and unfroze the moment.

Back in the present Stewie is in the morgue, the hospital recently declared the brain dead child legally dead and the parents of the hospital requested to cremete their son. The employee closed the cremation machine and thats when Stewie froze time. He opened the machine and dragged the child out and brang the child home. Stewie unfroze time and the employee turned on the cremation machine, unaware that the child isn't inside anymore.

Now Stewie has all the components he needs so he set to work. Brians personality has sucessfully been copied, now Stewie needs to implant that personality into the childs body. Stewie placed the helmet on the childs head and turned it on. Has the machine hummed to life the childs body began to shake and convulse as the helmet did its job. The personality implantation is complete and the child went limp. Stewie removed the helmet and kept a close on the child. The child finale opened his eyes.

"Oh my god, Brian. Your alive"

Stewie said has he gave Brian, who is now in the body of a child a huge hug

"Ugh, Stewie? What happened? Whats going on?

Brian now noticed his strange new human looking hands

"And what the hell happened to me?"

"Brian I can explain"

Stewie explained everything to Brian. How he went back in time to copy his personality to implant into the child of a brain dead child.

"I uhh. I guess I have to thank you Stewie"

"Sniff. Your welcome. I missed you so much Brian"

Stewie spent the next couple of minutes in Brians embrace, never letting go of the love of his life.

"Brian"

"Yeah"

"Do you remeber when I said I loved you, and you were willing to give us a shot during your last moments?"

"Yeah, I remember"

"Well. Do you still want to...Be togather with me?"

"Stewie I..Uhh"

"Brian please. I spent the last 2 month thinking of nothing but you. Thinking on how happy we could have been, the happy future we could have led. Now that your alive again I want to be happy with you now. You were willing to give us a shot, you said you wanted to feel loved Brian and I can give you that now. I have given you a second life and I want you to spend that life with me. I want you to spend this new life filled with love that I can give you, and the love that I always wanted to give you. And this time you don't have to worry about my age, your practically in the body of a 6 year old thats the same age as me. We also don't have to worry about society judging us, a child dating a dog. So please Brian, will you let me be your boyfriend."

Brian went silent thinking about Stewies offer. He remembered giving a relationship with Stewie a chance. All his life he wanted to be loved and he blew that chance in his old life, and now that he has this second chance at life he can't refuse this oppertunity he has now. Stewie just might be his only chance at love again.

"Stewie"

Brian held both of Stewie's hands

"I'll give this relationship a shot"

Stewie hugged Brian as tight has he could.

"I promise Brian you won't regret this"

As Brian hopped off the operating table he just now realised that he is a human boy...Naked. Now that he's human he has to try and get used to wearing clothes. Brian rolled his eyes and covered his penis.

"You had to leave me naked Stewie?"

Stewie said nothing. He just looked away shyly and blushed. He then tried to scratch behind Brians ear. But it did nothing.

"I am going to miss you as a dog. You were cute."

"Well, is there any chance you could... I don't know. Clone me a new body and put me in that one"

"Maybe. I could also probably infuse that new body with human DNA so it has the same lifespan of a human but I think I'd like to give this body a chance first, maybe enjoy this body before I get to work cloning your old one."

Stewie kissed Brians lips, and it didn't take long for Brian to kiss back. After making out Stewie pulled away needing some air. He then stroked Brians white hair.

"Your hair looks nice Bry"