I'd ask your doctor about it. They will know whether he'd be partly covered, and what the medical consequences of partial coverage would be if he caught the disease - maybe it would protect him from the worst symptoms or maybe it wouldn't (I am not a medical doctor and have no idea).

They can also tell you what the incidence of pneum disease is in your area, so you can get an idea of how high a risk you'd be taking if he isn't covered by two immunisations.