C.R.
I have teenage sons but have not dealt with this specifically. I as a 15 year old had dates with guys that were 17-18. I understand your concern about the age difference now but when they are both in their 20's it won't be such a big deal. When our 2 older sons were close to 18 we talked about how as soon as you turn 18 in the eyes of the law you become an adult therefore anything that may happen could have a very different outcome. We also had a similar talk when the oldest turned 21 reminding him that if he bought alchoholic beverages and shared with his underage friends he could be charged with giving alcohol to a minor with serious consequences.
Do you like this girlfriend? Do you feel like no one will ever be good enough for your son? If you feel no one will ever be good enough please re-evaluate how this will affect your son. My Dad's Mom felt this way about my Mother. It drove quite a wedge between them and also made us children feel like we could never really get close to Grandma because she was mean to our Mom. There was a time when my Dad did not talk to his Mom for a long time or see her because she had talked bad about my mom and he had enough. He asked her to stop talking badly to her and about her but she would not. My Mothers parents on the other hand accepted Dad as a son and treated him well. We have many memories of spending fun, good times with them as a family. Unless this girl is really engaging in bad behavior like sex, drugs, drinking etc... forbidding your son to see her will probably just drive him to see her behind your back. The other thing is when he turns 18 he will be able to move out if he chooses then you will have no control and possibly damage you parent child relationship. God Bless