I've had this with dd1 aged 7. For the whole of yr2 she missed 25 days through mostly non-specific viruses. She did have a week off for chicken pox and a couple of ear infections, her ears seem to be her weakness but having had them checked lots, no further action is needed. The rest of the time were just high temps and feeling very unwell. She eats well, lots of fruit and veg and has a varied diet, tho is very slim.
At the beginning of the summer, she missed another 2 separate weeks, I was just despairing at this point so I implemented some new strategies. I started putting her to bed at 7.30 every night, without fail (even weekends). She's terrible at settling and is rarely asleep before 9 and had previously gone between 8 and 8.30. I thought she would at least benefit from the 1 and half hour of rest.
I've been giving her a multivitamin (kids) with omega 3 in it, religiously. She's always had them before but had often missed days here and there. I've also made her drink more fluids, she barely drinks, tho likes water so I just keep reminding her to drink.
I took her to the docs at end of summer hols to check her weight and talk to doc about last school year. He said that it was a very bad year for viruses, her weight is very low for her age but took a view that the rest of the fam are v slim and were as children. He suggested seeing how she gets on this school year and maybe give her a blood test if it continues this school year.
Touch wood, we are now 13 weeks clear of any virus, which is a record. She did miss 2 days of sch but that was due to mild vomitting, poss caused by constipation. (She's only ever had one tummy bug in her life).
I can't believe how much better the year had started compared to last and I hope this will continue. We've slipped a bit with the bedtime and it definitely seems to make her a bit washed out, so will be going back to the 7.30 slot.
I hope this helps you and ds health improves, it's a worry I know. Def worth taking to docs tho, even if just to set your mind at rest.