Welcome!
A few tips for moving forward as you and your son expand your games:
+Consider picking up Tactical Operations as a PDF (hard copy seems to be a bit tricky to find right now). This book is chock-full of optional rules to enhance things, ranging from new weapons to new terrain types to whole new game concepts like artillery. Nothing in it is required for your games, but some of it can be very, very fun as a change of pace- for example, ever wondered what a battle looks like on a world with greater gravity than normal? Or on an ice field? Or in a swarm of locusts? Maybe all three at once? Welcome to TacOps. ;)
+Map sets. Up front, again, these are kind of hard to come by right now, but variety is the spice of life, as the old saying goes. The right map sets can give you terrain like giant mountaintops, dense urban morass, swamplands, spaceports, or even gladiatorial arenas! Worth hunting down and trying them out.
+I'm avoiding suggestions that move into the Battletech Universe's fiction and such, for now, because it sounds like you're enjoying what's going on in the tabletop for now, but a good bridge between the two are the Technical Readouts. They're the game's toy chest- each one is full of new units, refits of older units, etc.- I in particular would suggest TRO:3050 as a good starting point, because the stuff in it are popular, longtime favorite units refitted for a modern era... sometimes in a good way, sometimes hilariously flawed (which is FUN!). With the stats to use these units, background on them in the universe, and art showing them, they're a great gateway book. (Record Sheet volumes are best sought via PDF, and give you the actual record sheets for these units)
And of course, anything beyond that we can help with, we're happy to do so. You stand in the presence of some of the worlds' best Battletech players- and me too!- and we're always happy to answer questions.
Oh- and if it all feels overwhelming, well, there's 30-odd years of product behind this, and it can be overwhelming to even veteran players sometimes. Ask questions, we're here to help. :)