I think for a triage solution since you can't have him kicking your belly, walking away from the aggressive behavior may be your best option. Give him lots and lots if attention and praise when he's doing what you need him to do but walk away when he get aggressive. You can expect an extinction burst where the behavior will worse before it gets better. If he tries to approach you aggressively or cling on to you, peel him off, without talking until he calms down and give him lots of praise for calming down.
Praise for appropriate behavior and not attending to negative behavior will hopefully help you. Any talking (which should be very limited) should be in a very calm almost deadpan voice but praise for good behavior should be with big over the top facial expressions and voice.
I don't know what else you could do since the safety if your baby is in question.
If you have any questions or I can help you further let me know

@A MyAutismTeam Member we didn't know what set my son off either. It would start as anger at one thing and then keep changing what it was directed at. I couldn't bring myself to call the cops on my own child but when his father came to the house and tried to intervene, it got out of control and his father said he was calling 911. My son actually ended up calling because he didn't want his father reporting it, he wanted to give his version of what was going on. Turns out, it was the best thing that could have happened. We spent 8 hours in the ER but they did bloodwork, had him under security, etc. They they transported him by ambulance to an inpatient facility. We sat on the floor, terrified, hugging each other and refusing to let go. I just couldn't leave him there, my heart was literally breaking. After a few hours, they finally grabbed him and dragged him away from me. Both of us had to be restrained and i had a breakdown. Just typing this is causing me pain to remember. It broke my heart over again every time i had to leave after visiting hours. But my son got the treatment he needed and he hasn't had an episode since they got him on the right meds while he was in there. I hope no parent ever has to go through this but i know that's a useless wish so instead i pray that every parent has the strength to get their child the help that they need.

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A MyAutis...

Sorry! Meant to add - I hope your tests come back ok with the baby. You'll be in my thoughts :)

You can not react to this behavior. You need to stay calm and emotionless. You must walk away or turn away from him. This will work eventually. I have a 9 year old with the same diagnosis and I just got help with this behavior from our therapist. It is the only thing that works. The hardest part will be getting support from family and friends, they always try to step in, but the reaction needs to be the same from everyone. I hope this helps, I feel for you.

ignoring it or walking away is not always the answer. I tried this for a very long time with my son. It got to the point that he started hurting himself and threatening suicide. It culminated in a 911 call, a night in the ER and being admitted to a treatment center for 2 weeks. They put him on a mild dose of risperidone and he hasn't had a violent episode since.