With a JUCO he’s using up a year of NCAA eligibility. With post-grad he is not.
D1 coaches are always looking for pitchers who throw 90+.
If you choose the JUCO route, I’d advise being extremely careful as to which coaching staff you choose, and make sure they are on the same page as far as how many innings they think is appropriate for an 18-19 year old coming off of Tommy John. And do your research as far as whether you can trust what they say.
If it was my kid, coming off of TJ, I would do the post-grad or even simply a gap year. More time to recover, more time to gradually get the velocity back up without the possibilities of ramping up too quickly, and more time to mature both physically and mentally.
If he can get back to 90+, or even 88+, he will have a ton of options in competing with the 2022’s for recruiting slots.