Toddler not pronouncing words… help?

My baby is 16 months and there’s a lot of words that he understands, and quite a few that he can pronounce only the first letter of, but he doesn’t seem to be progressing past that. For example, ball, baseball, bat, bumblebee, belly, bubble, bath, are all “buh”. It’s the same with all the C words they are just pronounced as “cuh.” It’s been like that for months and he doesn’t seem to progress past that first consonant. Is that normal at this age or should I get speech therapy? He is progressing in the number of words that he understands, and slowly in the number of words he tries to say, but he is not progressing in pronouncing anything past that first letter sound. Is this something that I need speech therapy for, or do you have any recommendations that would help him? 

Oh also what number of words is it normal for a kid to try and say at 16 to 18 months?