Has he had Covid? I understand that can have long-lasting effects and remain in parts of the body. I guess he has been tested for glandular fever (mono?) which I believe does cause an enlarged spleen. The previous poster gave good advice about allergies. The doc to see for that would be a clinical consultant immunologist. Another thought is checks for possible parasites. I don’t know where you are based or how exposed your son might have been to parasites in the earth, wildlife, or certain foods if not cooked thoroughly. I hope he recovers soon, whatever the cause. You must be worried.
An enlarged spleen suggests he is fighting some kind of infection or maybe an allergy? I saw a case history on a TV prog where a woman - who had an enlarged spleen amongst other symptoms - was eventually diagnosed with a severe nickel allergy and had to have her nickel-containing braces removed. She recovered after that. A friend’s daughter was ill for years until she was eventually diagnosed with several allergies by a consultant immunologist in a U.K. hospital. Now she avoids certain common foods and is able to function again. She was pretty ill before the and going downhill. There are lots of people offering all kinds of allergy testing but I’d consider a clinical consultant specialist for a proper investigation if possible