Has something scared him while he was outside? Anything at all that made him fearful?
This really sounds like a fear reaction.
The best way to proceed would be to make sure you take him outside for a pee before picking him up. When he does pee praise him as if he's just won a gold medal in the olympics. Lost of "good boys" and a treat. Make sure he knows that this is what you want him to do.
If he pees when you pick him up, take him directly outside, give your pee command, and wait for him to do his thing. Praise, praise, praise.
Don't scold when he pees on you. Ignore it, don't react in any way.
Once he stops getting a reaction then hopefully he'll clue into the fact that peeing on you isn't going to get him anywhere.
In the meantime, any other household accidents should be corrected as usual. A firm no, then directly outside to do his thing. Remember the praise. The praise is very important.
He's still very young. A lot can happen at his age that can scare him into nervous peeing. Our lab was around 12 weeks old when our neighbor redid his roof. Our pup went out front to sniff around, and the roofers dropped some old shingles off the roof. It made a very loud sound. Our puppy was terrified, and for many months after that he'd pee out of fear. It took a while, a lot of patience, a lot of praise for the right behavior, and not reacting to the nervous peeing, and then he finally stopped.