85% of your immunity is in the gut - I'd get him onto some probiotics, Fast Tract would be ideal. Its organic, super potent and a range of strains of good bacteria, not one or two like others. Its called a probiotic superfood. Inliven would also be okay, but he might not be able to take it if he's not good with taking things!

https://www.bellybelly.com.au/misc/mivitality-inliven