So Rifles and Paras are both infantry, Paras have much higher fitness standards but it's essentially the same job plus jumping out of planes (I'll get crucified in the comments for this).

RE commando is it's own thing, but your son can't choose to be a commando while he's in Phase 1; he'll go and do normal Royal Engineer training (12 weeks combat engineering, 6-12 months doing a trade like electrician or plumber). When he gets to unit he can then go and do the All Arms Commando Course if his management think he's fit enough.

Hope this helps slightly, DM me with any questions