My dogs are the same. Esp. Gunther. He doesn't eat much of anything not only when boarded (with friends, haven't had to board him at a faciity yet) but also when we travel. When we moved from Ca to IL (with a long layover in MN for 1.5 yrs) he lost 5 lbs. He didn't eat hardly anything when traveling and didn't eat much the first few days we were at my folks......... .......and I was with him the entire time!
Of course you will be worried, he is your baby. But you can always call and check up on him to see how he's doing. I think having someone visit him would lift his spirits as well. Having something familiar there for him, bed, toys, etc always helps.