Kitten less active - could it be caused by low red blood cell or doxycycline?

I adopted two kitten brothers (Percy & Evan - approx 4.5 months) together about a weeks ago from the local shelter. The two brothers have been through a lot - URI at 2 months old once already. When I adopted them they are the size of 2 month-old kittens instead of 4.5 month old sizes. Two days after they arrived home Evan became sick with URI. Percy just had minor sneezing and nose discharges. I took them both to the first vet check - both are placed on doxycycline for URI treatment. Fecal exam was fine for both of them. But Percy has 28 red blood cell count, right outside of the normal range. HOWEVER I noticed after the medication Percy is a lot LESS active compare to his brother who is more sick. He became the lazy cat - prefers to sleep on the chair instead of playing with his brother. He naps so much compare to his brother! Its like his personality changed; energy turned off by a switch. The vet is only suspecting that maybe Percy is getting worse from URI, however clearly he is not. Both of them recovered pretty fast and only few sneezes here and there. Any reason why Percy naps a lot and less active? maybe from doxycycline? low red blood cell? He seems healthy and still eats/going to the bathroom the same.... Thanks for any help I can get here!

Low blood count would certainly do it. What did the vet do or suggest to bring the count up? Were any other tests or blood work done?

Are they eating, drinking and eliminating normally?