They introduced him, have him and John bonding and then it seems like the only time he's with them is during shootouts! Very disappointing.
He should have been living with them, developing more of a connection. Instead of sneaking around with Jesse *gag*

I feel like the rest of them should have got suspicious enough with his absences to mention it by now. They know terminators can make themselves look like anyone, so you'd think they'd be hypersensitive to any odd behaviour.
He just suddenly starts being gone all the time and the only time I can remember anyone mentioning it is Cameron making a comment about him being out running for longer than he should have.

It seems sloppy that they haven't caught on and checked it out.

I agree, it's sloppy of them not to question where he spends all his time. And it just seems like John would want to spend more time with his uncle, seeing as how he's the only other family he has besides his mom. And what exactly does John do all day now that he doesn't go to school? And does his mom know he doesn't go anymore? It seems like she'd be pissed.

Yeah. Especially since he's been distant from Cameron and pissy with his mother, you'd think he'd turn to Derek.
I know people could argue that he turns to Riley, but she (as far as he knew) didn't have a clue about terminators so he isn't getting support about that aspect of his life from her.

Yeah, but I think that's what he likes about her. Escapism. With her he can pretend all that isn't real.