Yes, he can go to jail. First of all, anyone over the age of 18 doing anything with anyone under the age of 18 in any sort of sexual, or romantic manner of any kind is strictly illegal. Some people say there's a 3 year gap (15 can date 18, 16 19, and 17 20) but this is not true. Even if that WAS true, he's FOUR years older than you, so it's just flat out illegal. I personally say age is just a number, and all the more power to the two of you, but just be careful, because he can get a jail sentence for it.