I would add Pepcid AC (Famotidine) also.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7552173/C, D, Zinc et al
How much horse paste?
I take about 3/8" (or 1/10 of the tube)...
BUT... you will have decide for yourself/your son... SUCH THINGS ARE A VERY PERSONAL DECISION.