Son, 17, has igg 850, iga 89, igm 17. levels are decreasing with each lab over last 18 months. Low response to pnuemovax 6/23. Could this be CVID?

No. It is called impaired anti-polysaccharide response . If there is recurrent sinus and/or lung infections, either prophylactic or gammaglobulin replacement may be considered. If he is healthy otherwise, it would be ok to just wait and see.