If you've been accidentally reinforcing a bad behavior and then you stop providing that reinforcement as part of ABA or other behavioral therapy, then often the child will escalate the behavior in the short term. Stick to your guns and then hopefully the frequency will start to decrease.

Still having the same issues after 3 weeks. He cries on the way to therapy. I am starting to feel sick when I take him because it upsets me so much. He seems to actually hate to go...but he has shown speech improvement.

Have you sat in on his therapy? I know it's recommended, but 40 hrs/week is very draining. He may need more sleep/downtime. I sit in on my son's sessions (only 4 hrs at a time) and I find it DRAINING! It's basically someone quizzing you the entire time (I know there's breaks-rewards), but it is intense. If it were me, I'd be cranky afterwards, too.

That wouldn't be unexpected. I would bring it to his therapists attention for their information.
Children with ASD can have tremendous difficulty with changes in schedule and new things, people, places, activities. I would reserve judgement for several weeks though it could take him even longer to fully acclimate.

If you just started the Therapy most likely the child will be aggressive due to it is a change, Aggression in some children is anxiety in disguise, ASD children do not like change, this is making him anxious perhaps. and that will create those issues you are seeing. Perhaps you may consider doing some sensory therapy.s with your child before you do the ABA and even during the ABA. any change is over load, on the mind and on there muscles, there are lots of idea I pined on the pin board for sensory..also look into Yeast over growth being a cause,, there are also many medication therapys to consider for your child, if you think that aggression/ anxiety is something that is a consistant challenge along with OCD behavior or ADHD on top of that.. track his progress and see your Doctor. There are SSRI's that could bennifit him.. the when the aggression is controlled you can see the learning sky rocket.