Does a 10 year old with a 18 year old sound okay to you ?
No it's not too big of a gap. The fact that he has a dodgy background is more concerning than the age.
I think the main issue between 18 and 26 is that you should be in completely different stages of your lives. He should be looking into settling down, forging a career, thinking about getting on the property ladder, etc... And well, you should just be starting out.
At the end of the day though, it's perfectly legal and if it makes you both happy, who is any else to judge? Although i do agree with those who has voiced concerns about his dodgey past.