"Oh, what a tangled web we weave when first we practice to deceive." - Sir Walter Scott
It's late enough now that I'll assume you dealt with the actual issue somehow.....but:
I know it is my fault for not checking his backpack
No. It's not your fault for "not checking his backpack." As you said...he's not a toddler. He is 14. Getting his homework done is HIS responsibility. I ask my kid when he gets home if he has homework. He answers yes/no. I drop it. I might, later, give a not-so-subtle "remember that you have homework to do and bedtime is at X:xxpm." Bedtime rolls around -- and it is off to bed with him, whether the homework is done or not.
You have my sympathies, though. My stbx was a LOT more...ummmm....enabling(?) when he was around and it made it really difficult but there were some issues about child-raising that I felt really strongly about and I didn't give a flying hoot. Dad will 'help' you with your homework (aka -- give you the answers?) No, he won't. It is YOUR responsibility, DS, we are BTDT on 8th grade algebra. Dad says you can stay up until you get your homework done? Nope. Mom says "go.to.bed...homework isn't done because you forgot about it? That sucks, but I bet you won't let that happen again, right?"
*sigh*....this is bringing back memories of my oldest DS who would come home and knock his homework out right away so that he didn't have to worry about it......
In my effort to be *concise*, I often come off as blunt and harsh. Sorry, don't mean to be offensive.