Teen boy constantly picking toenails and spots etc

Hi, my teenage son is forever picking at his nails, spots on his face etc to the point where he gets little infections, but refuses point blank to put any antiseptic cream on or let me take him to the pharmacist or GP. He is adamant that he knows best and will happily spend hours in the bathroom removing bits of skin off his feet, squeezing puss out of a swollen toe etc. 

I appreciate this isn't the most pleasant post, but does this resonate with anyone here? I just want to keep him safe and healthy, but it's a battle I seem to keep losing. 

Thanks