I found this info using a web browser search and I have no idea if it's viable or not, however your son may be able to apply for up to three years sabbatical from military service for a range of reasons:

https://www.blufmilitarybenefits.com/cip

It wouldn't surprise me if every request is rejected, but he could try... take a year or two or three off and then make a rational and informed choice when we absolutely know how deadly the clot-shots are.

If he does attempt this, I 10000% recommend doing many hours of research and speaking with veterans and servicemen who've recently succeeded in apply for military leave, so he can decide which "reasons" is more likely to be approved. Find online discussion forums for servicemen and ask everyone.