We have seizures in our family too - and you can have more than one type.
I also had a child who was misdiagnosed with autism at the same age as yours - so know what it's like to be confused and wondering if they will figure your child out. My child wasn't hitting milestones.
In the end mine had frequent colds and ear infections. So the fluid in his ears made his balance off (making it hard to stand or walk). His behavior changed - he would be rough/frustrated/tantrums - and he wasn't able to hear clearly - so his speech was delayed. I've known 3 other families who ended up at ENT in the end as well, that's what it was. So I just wanted to mention it.
My niece has absence seizures. You can be talking to her and she just starts staring. She is disoriented when she comes back at times. Most of the time she doesn't know she's having them. But she can be disoriented and upset.
I agree with everyone below. Film it. Record when it happens. Also - keep a log. Seizures don't always have triggers - a lot of the time they don't - but you may as well keep a log and just record when they happen and what's been going on. I just print out a copy of a calendar sheet off internet and record it when it happens. Anything he just ate, if he didn't get enough sleep, what activity he was doing, etc. All that is helpful when you go in. If you can see patterns, that will help you too.
Good luck :)