He could have ADD or it could just be age appropriate. There is an age range of appropriate behaviors & learning abilities. Just because some kids can do something at the younger age, doesn't mean that any kid that hasn't or can't do it, is behind. It's called a RANGE for a reason.
Yep, ADHDers can focus ALL day long on something that interests THEM. And they won't want to stop. EVER. Also meltdowns are pretty common for ADHDers, due to all the daily frustrations that build up & then once home, one little thing & they will blow. They've saved up a lot of pain & anger for where they feel safe & loved. Look at it this way, he feels safe & loved, you are doing your job!
For us, the testing involved a questionnaire that the parents & the teacher filled out. We did eventually go to meds for myself & 2 of our daughters, but pills aren't the cure all, more of an aid. Without daily exercise, consistent routines, a healthy diet (eliminate artificial dyes, flavors & sweeteners) & making sure that they get enough sleep, they still had issues with focus, anger & meltdowns. And they were going through puberty. And I was going through menopause. Oh Joy.
My youngest daughter has had wonderful teachers who "modified" the classroom for her (& others). Letting the students sit on a big yoga ball, so they could wiggle around, etc. And when she was in 2nd Grade & REALLY wiggly & talky, her teacher let her sit at the back of the class & anytime she felt the need to move, my daughter would get up & do the splits or stretch, etc. But she had to do it QUIETLY. How I LOVED that teacher!
My daughter had spent ALL of 1st Grade in DETENTION for talking & not sitting still. She would get up & try to help her classmates with their work, because she was way ahead of them. And then she would be sent to detention during every recess. Where she had to sit still & not talk. She was 6! Yeah, we changed schools. And she didn't need any meds for the ADHD until she was closer to puberty & the emotional roller coaster became overwhelming.
Maybe ask your husband to help you teach your son the different coping skills that HE is using to manage his ADD. Everyone is different, but chances are, your husband is the best teacher for your boy on this subject. Good Luck!