Given where you are I would assume that the baseball is a similar quality to what we have around here. Your son throws at a velocity very similar to our JV pitchers at this point in time including one freshman they've brought up. Personally I don't think any of ours will dominate week in week out at the JV level. For example, we played a strong team two weeks ago and every one of them was hit fairly hard. Their starter was throwing consistently 81 on a stalker and I'd guess maxing out at about 83 or 84. He also had good command and good breaking stuff. He was dominant for JV ball and that is what it takes to be dominant in our league at this point in the year. We played a weak team this last weekend and none of our regular pitchers was hit much at all.
We have a relatively weak team this season and should finish in the middle of our JV league at best. Our pitching is a little above average but nothing special for JV in our league. We've got one kid throwing 81 max and consistently in the high 70s with a good breaking ball. We've got one kid throwing about 1 mph less with better control and mound presence. We've got another throwing mid to high 70's, max at 79 or 80 and another that I haven't gunned but probably mid to high 70s max 80. Our injured pitcher would probably be throwing as hard as the the first or second ones that I mentioned with more fastball movement. I don't think any of those pitchers would benefit from being on varsity for more than a brief stint to eat innings during a tournament.
One year the coach kept a mid to high 80s kid on JV as a junior. He signed with a D1 before playing varsity ball and was the ace of the staff as a senior. He was a star in his freshman season at the D1 reaching the mid 90s. If being on JV as a junior didn't hurt him I don't think being on JV as a freshman is going to have a negative impact.
Unless you happen to be in a weak league for the area I think he's much better off playing JV this season.