Is it possible to have too little sperm?

Myself and my OH are trying for a baby and have been for about 5 months. Having lots of sex around ovulation ect. However the other day I gave him a blow job and when he finished his penis pulsed like it had cum, but pretty much nothing came out (less than half a teaspoon) this has also happened before. Can anyone suggest what it is? & how his sperm could be increased? And would this hinder our chances of conceiving? Thanks xx