By age 19 he should be able to retract his foreskin easily. It may be that the frenulum (the bit underneath) is too tight. This can be surgically corrected. DO NOT attempt to pull his foreskin back on your own. It can cause immense pain. If he has erections and the foreskin does not retract then this can cause issues in the future. If he has erections and the foreskin retracts naturally, then I think you have other issues to deal with.
If it won't retract at all, get him to see a doctor. Leaving gunk under the foreskin has been related to cervical cancer in women and penile cancer in men.
The foreskin retracts partially when he has erections, but not completely. Any advice for that situation? I'm a little lost as past boyfriends never had this happen.