How do you play in order Pokemon Diamond?

I'm pretty sure he already saw his mom in the Contest Hall in Hearthome City. You see her the first time you enter in the Contest Hall. This doesn't really matter; she only gives you an outfit to wear in contests (your son probably already has it). Anyway, you can't battle the Hearthome City Gym Leader when you first go to Hearthome City. This Gym is actually the fifth gym in the game. What your son needs to do is head east of Hearthome City on Route 209 (he might have to play in one contest first). Then he needs to head north on Route 210, and then east on Route 215 until you reach Veilstone City. This is where the third Pokemon Gym can be found. After defeating the Pokemon Gym and Team Galactic Saga in Veilstone City, head south on Route 214 and Valor Lakefront, and then west on Route 213 until you reach Pastoria City. This is where the fourth Pokemon Gym can be found. After you defeat the Pokemon Gym and Team Galactic Saga in Pastoria City, head back to Valor Lakefront. Cynthia will be there and will give you the Secret Potion. After this, you must go back to Route 210 and heal the sick Psyduck. After doing this, head north then west on Route 210 until you reach Celestic Town. After you have defeated Team Galactic and have talked to Cynthia's Grandmother, head back to Hearthome City. You can now face the Hearthome City Gym.

If you need anymore help from there, simply ask another question.

Hope this helps!