Imperial is ranked higher than UCL. But, to be honest, neither of them have a life. I had to a choice to go to both of them. In the end I chose Queen Mary because it appealed more to me and was great in the league tables in my course. Your son will have to pay higher accomodation fees and higher living expenses due to being in Central London. They're both great Uni's serving with great distinction. But I'd advise him to look into what kind of life he'll have.
I would agree with you to a certain degree as I have never met anyone from Imperial on a night out or any event etc, but i have met loads of UCL people! I go to KCL

Both very good Uni's i would probably choose UCL if it was me though...tbh i don't think the socials great at Imperial, but i could be wrong.