Watching from Olympus the treatment that was receiving his youngest son was difficult. As ruler of the Gods he had to be an example, so he couldn't help Harry. The worst part was that he had promised her beloved Lily than he would look after their baby, and he being tied to the ancient rules couldn't keep that promise. As such, when the war ended and he could get away from Olympus a couple of days without much notice from the other Gods he promised himself that he would visit Harry, he deserved to know who he was. But when Kronos had been defeated he found himself more observed than before, some Gods believing that the treat could have been stopped earlier if he had put more attention.
When he found out that his daughter Thalia had left the Hunt, he thought that he could guide her daughter to her brother. He couldn't interfere with mortal affairs, but Thalia wasn't bound to the ancient laws and she could. It wasn't difficult for the Lord of the sky to change the departure times of some planes, so his daughter got into one that went to London. With her daughter going to the right country, he sent a message to Morfeus so Thalia would receive the adequate dreams.
It was easier to concentrate on his duties knowing that his youngest would be out of the hand of Lily's horrible sister and in-laws soon. He continued to observe Harry's life, as he had done in secret since he was born, and seeing how the Dursleys were treating him worst than in previous summers. He couldn't wait till Thalia found and got him out of that place, he had no idea how Dumbledore had put his son in the care of those people.
The day had come, and he was nervous. The Dursleys had locked his son in the cupboard under the stairs, his old bedroom. His daughter was in the neighbourhood and had taken a look around 4th Privet Drive before retreating to a close by park.
As the night was falling and the Dursleys returned home, the weather started to change from the sunny day to a stormy evening. It was due to his son, his accidental magic paired with his demigod powers were affecting the weather. He must be in a really dangerous situation to create such a powerful storm, so he prayed that his daughter was in time to save him.
He was proud when he saw how Thalia had dealt this the mortals, even though he wouldn't have been so lenient with them. With them out of the picture, he observed how his daughter took care of her brother, and when he though his appearance wouldn't count as helping his children he appeared in the house.
The week after the end of the war, a new law had been passed with the approval of all the Gods. It was the first time ever that everyone agreed on something. The law was the following: 'As long as the God's responsibilities are fulfilled every God had permission to visit his/her children every seventh day of the week. The visit might include, if necessary, an explanation of the Greek word and claiming of said child. It is permitted to give gifts to the children and help them with personal affairs. Exceptions are as follow: said visit couldn't interfere with mortal affairs or demigods quests and the received help couldn't be made in a death or life situation.'
So following the rule, and seeing that it was still Sunday in London, he was free to visit his children and gave them all the answers that they will most likely have.