Hello dear!! This is very common problem. Few days back my father in law also faced the same issue. We have also done with all tests. Everything is normal. His hb levels are little lower. As they grew older, they immune levels, resistance power gets slower. But don’t worry. Stress free, healthy diet, light walking in fresh air, plenty of liquids could help him more better. Fever could be seasonal change, Infact sometimes they gain resistance Which more important at this age to fight with foreign particles. Stay strong.