You can subscribe to Match.com and other sites for up to 12 months. However, if it shows his account has been active in the last 3 weeks, he has probably logged on, but not regularly.
Possibly, he's still logging on just out of curiosity. I've been on similar sites and have logged on ocassionally, out of boredom, but when I met my boyfriend, I either deleted profiles or deactivated them from search lists.
Honey traps aren't a good idea, as you might find out something you'd rather not, so instead of trying to entrap him, you should ask him outright and tell him you are unhappy about him still being on the site. You don't even need to tell him you were checking up on him, tell him one of your friends saw him there.
If he really cares for you, then he will delete his profile, if not, then you have possibly become involved with an internet serial dater, of which there are many.