My primary opponent is my 11 year old son. We currently have the original core set, 1 x-wing expansion, 1 a-wing, 2 tie fighter expansions, 1 tie-advanced, then scum and villainy. I'm planning to get Heroes of the resistance since that's the Falcon he'd prefer to fly. After that, what would be one or two purchases to keep our games so-so balanced? I realize we are going from early-wing to a more recent release, but any ideas would be appreciated.

Heroes of the Resistance is a great set. For casual play, it's not a bad idea to pick up a T-70 expansion to make good use of all the quality pilots you get with HotR. Or maybe a TFA core set, which also comes with BB-8, which is both a good card and a recognizable character, and will give you a couple of TIE/FOs which are solid.

The Upsilon is a fun and good-looking model. Kylo Ren is maybe not a super fun pilot for an 11 year old to play against (his ability can verge on un-fun) but the others are more accessible, and it comes with a bunch of interesting upgrades. It's fun to fly and my son thinks it's the coolest model I own. It's definitely a presence on the table.

A good small ship to add might be the Inquisitor's TIE. If you son watches Rebels then it's a ship he'll know, and it's fun to fly, with interesting pilots and a really good dial. Depending on which pilot you use it can go from a quality line ship up to a very slippery and hard-hitting ace.

Which scum do you have? Without knowing that, i would say look towards the imperial veterans or a decimator to help beef the empire side. Maybe a Tie FO blister to get Omega leader (and everything he needs)

We have the S&V 'starter' set with the y-wing and the two z-95s. I have been thinking about the Inquisitor's tie. thanks for the prompting.

Your money and your will. My 5 cents...

If you are going to play with your son I would avoid 360 degree firing Large ships. Stick to Small ships and enjoy outmaneuvering your rival. I love S&V box. Even better with the ships you own. Y wings and Z 95 are also Rebel ships so you can proxy easily.

Tie Interceptor and Tie Advanced Prototype are super funny ships to play with a young pal. There are many good Interceptor pilots. So I would invest 30 $ with these.

Then proxy Imperial Aces, Z 95 and Y wing Rebel expansións. And tons of fun. If your son continues playing he will obtain someday this expansions.

Yt 1300 can be a little boring. Or Yt 2400. Or Decimator. Small ships are cheaper and more skill involved with them.

If you have Scum and Villainy then you can use a Firespray with either Imperial or S&V lists. It's not great as an Imperial ship but it's good enough, and there are some interesting Scum pilots for it. It has fore and aft arcs and an interesting dial, you can kit it out with a cannon or as a bomber, and it's an iconic ship. And everybody loves Boba Fett.

I had been thinking about the Decimator but I ended up thinking along the same lines as Hexdot -- you already have a 360 arc on the Falcon, you probably don't need more of those.

Another recognizable ship that you both can enjoy is the TIE Bomber. Adding obstacles that blow up adds to the challenge of flying, which I think is fun.
The ARC is another ship that has a lot of potential for fun. I don't know if you like to stick to generic pilots for the sake of simplicity, but the ARC doesn't have generics, they're all named pilots.
A second Y-wing wouldn't be bad either. B-wings are cool too.

If you want to expand your Scum, the Protectorates are pretty solid: they hit hard and have high mobility.

For small base ships, avoid the Khiraxz, the StarViper, TIE Punisher, & E-Wing. They're either too expensive (points-wise) or fall short of their potential. They're fun to fly--I like them all--but you don't really need them if you want to keep things streamlined.

I find that almost any ship you want is a good ship to get. They are all pretty well balanced and fun. There are definitely some super combos you can get for certain ships that really make them a bit crazy, but for the most part, if you don't build that combo, it's alright. Let's take Parratanni, for instance. It's a powerful tournament list, but if you take just any one of those ships and build it a different way, it's not so broken.

The one exception to this is the Tie Phantom. It's a bad ship for balance. If you get one of the named ones with Advanced Cloaking Device and Veteran Instincts then there are just a few tools you NEED to have to beat it. If your list has those tools, you win. If you don't, you lose. You might as well play Rock, Paper, Scissors.

Main reason I suggested the decimator was for a Falcon vs Evil Falcon type game. Yes I know they fly differently.

The firespray might be pretty cool for casual games at home.