Mentally underdeveloped son reaching puberty, please help

Hi 
I have a son with mentally undeveloped,  he is 16 now. He is getting matured feelings as a boy and he also have got hair at private parts. It is very hard to think and making him habituated to activity of hair cut at private place. I don't know what to do. He is not allowing me to cut the hair. Can you please help me on this what I can do.