Well, I just had a thought. You probably have enough nukes now. 2 or 3 per enemy city is enough. Now you need to build up your conventional forces. By this I mean Modern Armor and Transports and Battleships. You should build 1 MA per enemy land combat unit, 1 Transport per 8 MA, and 1 Battleship/AEGIS Cruiser per Transport.
Once you build all this, you position it just outside his territory. Next turn, when you have all your movement points, move the transports in and land tanks on his territory, around his cities, but not adjacent to them.
That turn, he'll ask you to get out, but won't auto-move you. Your following turn, declare war and nuke each city at least twice. This will kill off about 3/4 of the defenders and leave the rest so weak they are unable to resist. 1 MA should be able to kill off any enemy unit, and possibly several. Also, the nukes affect the squares adjacent to the impact zone, so that's why you don't get next to the cities.
The Nukes are your artillery. No need for Arty or Bombardment; that's not what the BBs are for. They are to plant on the transports to keep them safe. Once you've nuked a city twice, take it. Do what you want with it. Razing is best, because think of all that clean up and rebuilding you'd have to do.
If you get ALL of his cities in ONE TURN, he CAN'T counterattack.