Honestly, while there are options on FS bikes if the rider is confident enough (I used spacers on the forks and offset bushings on the shock to lower standover), on a rigid bike if they’re struggling to straddle the crossbar, then the bike is too big for them right now and, especially if you intend to ride offroad, they’ll only end up injuring themselves.
Regarding saddle height (which is a different thing) and reach, these can be adjusted according to terrain, skill and rider height – i.e. maxed for the road, relaxed for the trail – and kids taught to start and stop from the crossbar, but this assumes a frame that is small enough to allow them to do this.
I’m sure that you have a pretty good idea already as to whether you’re pushing him up a bit early…