Take him to an occupational therapist and put him in twice a week therapy regardless of whether you also see a dev pediatrician.

Also give him Nordic Naturals fish oil and read about the Feingold diet. Leave no stone unturned in finding help. It's not too early to start finding help.

My son was around the same age and the dietary changes helped quite a bit.

Later, when he was older we had to put him on ADHD meds and it helped with the impulsivity.

Yes!

OP here - what do people think about follow up with Belsito? Should we skip and go to a child behavioral psychologist/other? I am confused.

Yes that could be. The course of action would likely be similar no matter the diagnosis. Alford baker in MD maybe? Stixrud? Also consider that if you are remaining neutral/no reactive when he throws things he may be ramping up the behavior until you do react. It's called an extinction burst and can precede a reduction in the behavior when he sees that you will not react with emotion no matter how much he does it. You might find the Ron Leaf book A Work in Progress (?) helpful. Behavioral principles are explained and there is a chapter on toilet training and other behaviors. Also does he throw things at preschool? If not you can ask the teachers what worked for them.