yes, autism is hereditary. we were scared with abby since dh's other son is on the spectrum, but then his ex had another baby with full blown autism, we figured it came from her side.
i cannot stress enough getting him evaled asap and help asap. go momma bear on DH if you have to. i know kids zack's same age who are doing twice as much as he and aren't as high functioning (like not being fully potty trained and he's 10). good luck!
also, adhd and autism lots of times goes hand in hand. Zack is the same way, though it depends on the environment at home too. since we sent him to live with his gma, he's now running on counters, screaming and punching teachers and the like. not much we can do 100+ miles away, but definitely consistency at home helped when he was here.
any questions, let me know!