Sounds like a developing/borderline frotteur. It sounds intentional since he made no other advances and that you mentioned he's not a virgin.
http://www.emedicine.com/med/topic3439.htm
The DSM-IV lists the following diagnostic criteria for frotteurism:
The patient experiences intense, recurrent, sexual urges and sexually arousing fantasies involving touching and rubbing against a nonconsensual person. Symptoms must be present for at least 6 months.
The patient experiences significant distress or impairment in social, occupational, or other important areas of functioning because of the fantasies, urges, or behaviors; or the patient has acted on the sexual urges.
Frotteurs typically act out their fantasies in crowded places (eg, public transportation vehicles, busy sidewalks), which allows for escape; the frotteur can claim that the touching was accidental. The frotteur rubs his genital area against the (usually female) victim's thighs or buttocks, or the frotteur fondles a woman?s genitalia or breasts with his hands. While committing the act, the offender typically fantasizes about an exclusive, caring relationship with the victim.
Most acts occur in perpetrators aged 15-25 years, after which frequency gradually declines. Frotteurism has been noted to be equally common among older, shy, inhibited individuals. Fantasies of frotteuristic behavior without action have been reported as a stimulant to sexual arousal.
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I've read of cases of frotteurs who would use inanimate objects as masturbatory aids by rubbing up against them. So, while the typical frotteur does it in public/crowds with people, it varies.