eh, i had so many guys purchased from my first army that i didnt know jack about.. i dont regret. learn what you can with what has brought you in and then go with it. id rather learn to paint on something random then worry about getting it right the first time. especially when a lot of stuff resells on ebay for a decent price!
So you have one tactical squad, one terminator squad, commander, and dreadnought. its a fine starter mix if you ask me. if you and your son intend to play together often, dont upgrade your army without helping him upgrade his. i dont know how old he is but a tweaked army compared to AoBR orks will crush those orks every time.
you could look into getting a command squad for your commander and the rhino or razorback transport for them or your tactical marines. Rhinos you can transport 10 guys and 2 can shoot out of it, razorbacks have a 6 man capacity and heavy weapons mounted for shooty. I would think about getting a different arm for the dreadnought, because i'm partial to the assault cannon as a good quick fix. Multi melta dread has a very short range and at such low points is a large target with a very specific/limiting function. (or not though, run with it if it fancies you!)
If you we're to help your son out, get him some ranged options for his orks, lootas, big gunz, looted wagon with a boomgun, something long range shooty to support his close combat ork boys.
oh and get the current codex for space marines and orcs. i personally couldn't live with just the AoBR army lists when thinking of expanding.