Both of my kids are super into baseball, but had friends who tried it for the first time in 5th grade. Totally doable!

Two things I would do if he's truly never played- agree with the poster above, play catch this winter when you can. Just practice stepping and throwing with decent form and accuracy. As someone else pointed out, a LL will likely have pre-season evaluations, so that coaches can rank players for drafting. It doesn't really matter how he does, but he'll likely feel more comfortable and be safer out there if he's competent in throwing and using his glove.

Also, think about reviewing some BASIC baseball rules, to the extent he doesn't already know. When a kid starts out at majors or AAA, the coaches may not think to explain some of the beginner baseball stuff- like don't just run on a fly ball when there's less than 2 outs. Knowing what a force out is. No need to dive deep but I always feel badly when a new kid is embarrassed about making an error because he just doesn't know something simple.

If you're thinking about your local LL, you may just want to reach out to someone now to see what level they would recommend placing him at. At least in our LL, they have a good idea of what kids they'll have at AAA and majors, and where a new 5th grader would fit in.