This is just personal opinion, so ignore me if you like.
Please remember that these are sports cars. Look in the newspapers any
week, and you'll probably read about another terrible road accident
which killed another 17 year old. Best to buy another car altogether and
buy the MG a couple of years down the road, when your son has a bit of
experience behind the wheel. It doesn't have to be boring, far from it!
My first car was a mark 1 Ford Fiesta. It was very cheap and taught me
more than any workshop manual, owners club magazine or mailing list ever
could about cars. It is great fun to drive - the words "Mini-roundabout
Ferrari" spring to mind! I still use it today. I'm not trying to put you
or your son off, but (as I've already said) this is just personal
opinion. If you still want one of the cars you mentioned though choose
the MGB. The MGB mechanically is far simpler and stronger than the TR7.
The build quality of the TR7 was pretty suspect, to say the least. It
would be cheaper, though.
1967 MGB GT