What he’s doing is very much a cornerstone strategy that most Russians should do when fighting against Germany; consolidate a ton of troops in West Russia and use it as a staging point to contest Belorussia, Karelia, and Ukraine. Be extremely careful of your attacks and where you leave troops after them; Russia’s positioning is excellent for reinforcement from West Russia and Caucausus.
The best overall counter is to basically mirror him; consolidate your forces in Eastern Europe and abandon everything else on the front except leave 1 infantry in each location. What will happen is you both fight for and exchange territories every turn. This is the best you can do if the UK/US are pressuring you from the west. Your fighters will give you a great advantage in retaking territories; send infantry + fighters to retake the territories. Try not to send tanks as you will lose them in the counterattack. When you can, start upping the bar by retaking the territories with more infantry. Russia will run out of men before you do and he shouldn’t dare to send a large force into one of those territories or he’ll be counter attacked by your own huge stack of tanks + fighters + infantry waiting and he’ll be too far to reinforce the attack.
Germany needs to get into Africa to get the IPC edge needed to push Russia well. If the Allies insist on keeping you out of there, then just play defense; you will never crush Russia alone.
If the Allies aren’t pressuring you too much, start building stacks of tanks and march your infantry eastward and simply try to overpower his forces. You can pretty much build 8 tanks per turn while Russia is building 8 infantry; combine with the momentum of your fighters and initial tanks and you should be able to outdo his cluster of troops which you will absolutley need to attack at some point.
I would not recommend putting all your troops in Ukraine. It is too close to everything that Russia can pound you with, and you also cannot spare fighters to simply land there for defense; they must be used elsewhere to throw the UK out of the mediterranean and go into Africa. The best you can reasonably have in Ukraine would be 6 infantry and 6 tanks, which is going to get annihilated by a combined attack from West Russia (11 infantry + 2 art + 4 tanks) and Caucasus (2 fighters + 6 infantry; you cannot deploy 6 infantry in Caucasus but you can move the ones from kazakh into there to combine with the 4 you did deploy there). You would need nearly all of your fighters in Ukraine if you wanted to try to keep it from the first turn and onwards, which is very stupid since it takes a couple of turns to get more troops there while Russia is building right next to you.