(Disclaimer: I am a secondary school teacher, but I have taught elementary-age kids in the past. While I know how to teach and have experience with kids that have attention issues, I am not a trained educational assistant nor do I specialize in children with additional needs. Ultimately, your child is your child and you will know better than I do about his temperament and preferences.)Starting with introductory rules, as mentioned, would be best, but ensure that you are gradually adding aspects as he learns. For example, start with shooting since it is all about rolling dice, getting a sufficient number, and blowing stuff up. Then add more as he begins to master it, rather than read all of the rules at once.
Activities such as rolling dice as key. Active learning will be important for getting him involved and feel like he is accomplishing something. Pencilling in bubbles will be big, but I will say that being involved with the rules may be most important. Having him listen for a while may not be his cup of tea. Have him help you read things out, pick out 'Mechs, etc. If he feels like changing a rule, let him! Kids love their own versions of board game rules, no matter how silly it may seem to adults.
Besides being active, give the game meaning. Focus less on the mathematical part of the game (sneak it in instead!) and more on that it is about big robots shooting at each other. Get silly and make explosion noises to ensure that a success feels awesome! As others have said, kids love competition and getting to beat their dad at a game. And if he gets a little bored, offer a break. Don't force it if need be. Most of all, be encouraging and foster a belief in his abilities.
Good luck. You'll do great!
