Other, more experienced players playing as the Germans might be able to help better, but, I'll still take a stab at the problem.
It'll help him to prioritize getting the tank upgrade from heavy cavalry on priority.
Till then, he can probably try the following composition (assuming his troop capacity is already 140; if not, reduce 5 riflemen from the composition if Standing Army chapter 5 is pending; if all three barracks haven't been upgraded, we'll have to play around with the composition a little more; let us know, and we can try suggesting alternatives):
6 heavy cavalry
5 artillery
3 supply carts
38 riflemen
Assuming he's going in for war attacks and other key attacks after earning at least 1 star from a previous battle; else, please suggest that he can go into a multi-player battle, and get at least 1 star to get the Teutonic Fury (+15% damage boost) active on all troops for the next attack.
Since he's apparently already good at using rally, he could continue using them the way he's comfortable with it.
It'll also help him to upgrade the oil refinery on priority, and build the air-strip.
In parallel, let him get going on the Aviation chapters at the library, at least till chapter 3.
Starting with 1 bi-plane, and stepping it up to 2 with the library research will help in keeping troops alive in battles - whenever his troops are about to get overwhelmed by defenders, sending in the planes should help.
Thereafter, getting factory to level 3 to unlock the Heavy Tank can be the next priority. In conjunction with this, it'll help to get the splash damage research for heavy tanks done at the library.
Teething troubles are common, and as you've said, it should be temporary. For gathering resources, at the moment, he could focus on attacking EA bases, and skip the more difficult bases. At least till he gets the tanks unlocked at the armory. Keeping the medal range lower will also help.
In wars, you can probably help him with identifying alternate targets which are relatively easier, till he finds his groove with the Germans.
Feel free to reach out to us with feedback on how this goes, and we can alter the suggestions based on the feedback.
Appreciate you helping him, by posting on his behalf on the forums! Cheers!