2 year old and Dog

Please help! My son is 22 months and we have been trying to teach him to be gentle with our dog (a female black lab mix) but no matter how much we show him, move him, separate him, or reprimand him he is still too rough with the dog. Today the dog actually snarled/growled at him and went to nip him. Getting rid of the dog is a last resort but I will never be able to forgive myself if she does something to my son. We also have another one on the way in 7 weeks! Please help with any advice that you have.

TIA!!