I haven't experienced this problem, as my
LO is only 15 months. However, I would be tempted to take your son visiting more often. Could you prep their house? Keep the toys out of the way, and make a big show of the cousin (?) offering him a toy to 'borrow'??? Praise the other child to the heavens and perhaps he might learn from it? Give the other child big cuddles for sharing. If your son then shares, do the same to him. If he doesn't share (more likely) then try your best to look disappointed. Take it off him, but don't then give it to the other child, just put it out of sight. Praise the other child again for knowing how to share, and say that one day
LO will learn, when he's a big boy etc etc. It's amazing how much they take in. Sorry if this sounds like a pile of naive poo!
xxx