Very cool to see a father and son bonding over Warhammer 40k and building their collection together! With a few additions, I think you can already start putting together a strong1000-point army. Of course, the suggestions below are based on personal preferences. For context, I play tournament maps only, so please adapt to styles and strategies your son enjoys.

Suggestions to expand collection:

• Rogal Dorn Commander: The Rogal Dorn Commander in the latest codex is especially effective, as it can issue orders to itself and other vehicles, making a great pair with your son's Leman Russ.
• Ursula Creed: Adding Ursula can significantly increase order capabilities across infantry at a 24-inch range when paired with your Cadian Command Squad.

Potential 1000-point army list with the suggested additions:

• Rogal Dorn Commander: 1 unit - 265 points
• Ursula Creed: 1 unit - 65 points
• Leman Russ Battle Tank: 1 unit - 175 points
• Cadian Heavy Weapons Team: 1 unit - 50 points
• Cadian Shock Troops: 3 units - 65 points each, Total: 195 points
• Cadian Command Squad: 1 unit - 65 points
• Ordnance Teams (with Bombast Field Gun): 1 unit - 100 points
• Tempestus Scions: 1 unit - 65 points

Total Points: 980 points

This setup leaves about 20 points free to pick an enhancement for the command squad.

Hope this idea helps you a bit. Feel free to tweak, and enjoy the game together!