I have often advised similar people to take their test and go for the full licence not the restricted ones.
Then be realistic and buy a suitable bike afterwards - nothing larger than a 600 until you get the hang of them.
Whichever route he takes for the test it will be expensive but it's easier to take one test rather than putting yourself through it all again to upgrade later.
When my stepson turned 17 I pestered him to take his test. I do remember him saying stuff like "Yeah yeah quit yer moaning", it didn't take too much to get him through. He then spent several months lauding it over his mates cos he'd passed his test and they hadn't..... Exactly the same story with his car test.
Now that the test has got harder (or at least more obstacles in the way) he is glad I whinged at him. His mate who is just about to turn 24 is now going to take his DAS as he is finally fed up of riding a 125. It doesn't help when he's got two of us saying "You should have listened all those years ago"
I do love it when I can say "Told you so"