You played it correctly. The good news is, a "loss" to your son is really a "win" in the long run!
I was actually quite pleased with how it turned out.
My son is only seven, so I always choose & place my army first so he can get a look and plan out his strategy. I'm not a Johnny Shotgun fan at all and when he picked him, I chastised him a bit.
I was quite shocked when my son activated him and blew away pretty much my entire force. Game was over on my sons's third activation when Syvarris mopped up and finished off the Hive...I didn't make it out of the first turn.
I thought I'd have a few more years before the worm turned.