In a perfect MBTI world, I echo the shallow part.
The thing would be, as long as you kept your subject alongside a) his subject, b) his direction, and c) his interests; you'll be fine.
For example, my ISTP brother LOVES Korean pop, and Korean (Asian) movies, and as long as I keep the debates rotating around it, I'm fine.
However, the rest.... the simpler word is shallow.
To be more intricate; he has less interest in general, less willing to go in depth, less willing to argue over 'what's commonly proven' or 'what works' (as it is with other ST types), and not willing to be bothered in general. "Meh, what a drag. This works, right? Why so complicated?"
Also, when regarding things he doesn't interested in, so far I see that ISTPs tend to take the path of least resistance. Not the path that needed to be taken. Their Se makes planning for the future almost nonexistent except for the sudden acute moments of it, in which they can't be stopped-- no matter how it may disturbs you or everyone else, because IT IS IMPORTANT. Sometimes that resulted in laziness. Sometimes that resulted in a devil-may-care, "eeh whatever, as long as it gets done, I'm fine.", sometimes that resulted in a sudden outbursts of "THIS NEEDS TO BE DONE DAMMIT WHY CANNOT YOU SEE".
In short; cannot be bothered.
There are definitely traces of it in INFJ (or at least, my own) personality type, which can make it worse via projection; it can make it look as if the ISTP in question is being particularly bad / uncaring, when it's actually their own personality.
Also, very very low emotional depth. Fe inferior = a certain resistance towards 'stronger' social connection, which....is alright if that's what you're seeking.
Again, this is my experience. Caveat emptor.
If you want to know what do you mean by shallow... How long have you two been going out?
When it's past the time of 'showing your best', ask him about things other than what he's interested with. You don't have to go for specifics.