Get him to make 'em up!
Start off with skirmish – humans move 6" a turn, bows shoot 30", say. Base other races and ranges around that. Get him to come up with movement rates himself. Are Elves faster or slower than humans? How about Orcs, etc?
Rate each race for toughness (how many wounds they can take before they die) and maybe give bonus wounds for heroes.
Give each weapon an attack and defense rating of 1-6. Basic sword might be attack 3, defense 3. A great sword might be attack 4, defense 2. A weapon in two hands might be 4 attack, 4 defense, etc. Get him to think about which weapons look better for attack, which for defense, etc.
Give each weapon a to-hit rating, again on a scale of 1-6.
Assign armour classes of, say, 1-4.
To fight, both players roll a d6. The attacking fighter (his round) adds the weapon's attack rating; the defender adds the weapon's defense rating. If the attacker wins he gets a to-hit roll. If the defender wins, the attack is beaten off. If it's a draw the defender gets to make an attack out of turn.
On a to-hit roll, add the weapon's to-hit rating to a d6 roll and then subtract the armour rating. If the score is more than, say, 3, it's a wound; if it's more than 6 it's 2 wounds. Anything less than 3 is a miss. A 1 is always a miss. If a six is rolled and it's still a miss, roll again – a 5 or 6 is a hit.
Give heroes 2 hero points to play with each combat round – they can add those points to their weapon's attack or defense, or to their to-hit or armour ratings as they choose.
For shooting do the same thing, but instead of making a weapon attack/defence roll, the shooter has to roll higher than the enemy's armour rating on a d6. You can add modifiers to that (-1 if moved, -1 if over 12 inches, etc).
Then shooter rolls a to-hit roll if it's a potential wound.
Turn might be move, shoot, fight if in base-to-contact with an enemy. Then it's the next player's turn.
Victory conditions might be rescue the maiden, kill the enemy boss, etc. Maybe start off with three figures on each side, or one side with 3 heroes fighting off lots of weaker enemies.
If he gets into it you could start adding other factors in as well – dexterity, fear, magic weapons etc, etc. You could also simplify the steps for mass battles.
Get him involved in making the rules and he'll be hooked for life :)
Good luck
Cheers,
Aaron
