Oddly, I just finished a game with my 6 year old this afternoon! He still has some issues sitting still and needs to learn some tactics, but if I ask him he can find all the stats on the cards, and can measure out his own moves. He still needs help with remembering all the stuff that goes into to-hit mods, but other than that he keeps up pretty well, and really likes playing. He has a command lance, and his sister has a recon lance. They have a hard time fighting those two head-to-head so I was adding in mechs from the assault and battle lance packs to even things out. The assault and battle lance packs are solid choices, the only down side being they are a bit slow. The striker, command, and support lances have a wider variety of speeds. Other than recon which is all speed and no armor, all the packs are reasonably balanced. The recon is great for a source of annoyingly fast mechs to add to a force though.
Now, a couple things that came up in our games:
With small numbers of units, say 4v4 or less, it helped to use the variable damage rules. Basically, you roll for each point of damage separately. Otherwise, light mechs were just a bit too vulnerable to lucky hits. It slowed the games down an extra turn or two, but toned down some of the frustrating instances where a mech would blow without getting a chance to really do anything.
My boy gets bored with any sort of scenario that doesn't allow for lots of carnage. He doesn't like stuff like Scan the buildings for intel or protect the VIP. He is more engaged with stuff like breakthrough, king of the hill, or just straight up beat each other to death for five turns and see who did the most damage. Maybe you will have a different experience, but my little guy wants to imagine robots exploding, and gets bored if they are not exploding regularly!
Lastly, if your son gets a little tired of painting, the Mechwarrior Dark Age minis are a decent source for pre-painted minis that you can generally find stats for on the MUL. I bought a few of the Solaris VII box sets and my boy loves them. They are much bigger than the lance packs, and all the metal minis, but still close enough that they don't really break the game if I let him mix those in.
Anyway, good luck. Hopefully it holds your sons interest.