Neem, Reishi mushroom and vitamin D. About 18 years ago, I had a virus that I just could not shake. It settled down in my neck ~ my neck ached (no tonsils, those got yanked when I was 4) and just would not go away. I felt just icky all the time and it had been going on for about a month. Then I ran across an article about Neem having antiviral, antifungal and antibacterial properties and Reshi mushrooms boosted the immune system. I went to the vitamin store that day and bought a bottle of each and started taking 4 capsules of both every couple of hours. I took the bottles and water to bed with me and every time I woke up, I took more. Within 48 hours I was coughing up green phlegm. By the end of the week I was finally over it. I keep neem and reshi in the house now for colds and flus. A year or so after this I had my vitamin d levels tested and they were very low and deficient (11 ng/ml ~ optimal is between 65-80ng/ml). So I started taking 20,000iu daily for 3 months until I got my levels up to 65ng/ml. Since then I keep it there by taking 5,000iu daily. I haven't had a cold in years. The last time I had the flu was in 2008 or 2009 when the swine flu hit. I was over that in about 48 hours.
Vitamin D researchers recommend when your sick that you take 20,000-50,000 iu a day vitamin d short term while you are sick to help boost your levels.
At the very least, he should take 10,000iu a day while he's sick and then drop back to 5,000iu.
I usually buy Neem by Nature's Way because that's what is available at the vitamin store. I just checked amazon.uk site and they have neem and reshi both in powder and capsules. He should take the neem and reishi capsules every few hours around the clock until he starts to feel better which should be a few days and then he can cut back to 3-4 times a day until he's well. I learned that if you stop taking them too soon, you'll get sick again.