I don't think there's a cause for worry. Mine is 10 months na alitoa za chini at 7 months then za juu zishatokea halfway. Wa jirani alitoa za chini at 1 year na two months later za juu ziko bado. Lakini @Luther12 ndio anaweza fafanua hii maneno authoritatively
Babies are supposed to have one tooth every month from 6 months onwards. Meaning at 1 year they should have 6 teeth, 2 upper and 4 lower. However, there's a great deal of variation depending on different factors including genetics. Most babies adhere to expected schedule but some don't. You should only be worried and see a paed if at 18 months he still hasn't shown signs and if he also have issues walking.