I think my son is going to propose to his girlfriend?

My son is 22 years old and his girlfriend is 36 yes its a huge age difference but i had accepted it my son met her in college last year and they just hit it off. Shes divorced with two kids and my son is great with kids he has a little sister shes 9 but anyway i think he is moving too fast with this relationship. My son has been asking me questions like how did dad propose to you my husband died when my son was only 5. I think that he is going to do something stupid tonight and regret it for the rest of his life he has a good job lives in an apartment but i still think hes not ready for marriage how do i stop my son ffrom making a big mistake?