Good idea on taking him to the vet! When in doubt always contact a good vet!
With that being said, both my girls had almost triangle shaped baldness behind both eyes. I have had them about a month now & with a greatly improved diet, (50% protein, 25% veggies, 25% fruit each day) more enrichment, (a larger cage, toys, out of cage time) both of their baldness is going away.
So, get him to the vet asap ... but, try not to worry too much! ;-D