I think this is a fairly widespread problem. I know that I and several of my friends have younger brothers who went through (and in some cases are still going through, even at 25+ years of age) something similar.
Based on my experience and observation I'd say the following:
Try to avoid going the heavy handed you-have-to-make-something-of-your-life route. There's a strong possibility of just getting his back up and so drawing out the whole situation.
I hope this question doesn't offend you too much but do you really know your son as a person? If you know him well there's more chance you'll be able to understand his attitude to work and help him with career choices etc. Does he know how concerned you are about this? It would be useful if you could ask yourself some searching questions as well to discover exactly what it is in this situation that bothers you the most and makes it difficult to accept. Why do you feel there is a need for him to push himself to acquire new skills etc.?
Would you be content if all he ever wanted to do was work in a pub/shop (these are the two jobs which come to mind when you talk about part-time work).
Are you supporting him financially? It sounds like it as I don't think anyone could be living on the money from a part-time job. While it's fantastic that parents can support their children by giving them a home and paying for bills etc. it's also important that this isn't taken advantage of. There are a few threads on when children should start handing up money at home and how much etc. which might be interesting reading for you.
Would your son be interested in travelling at all? If he has no particular aspirations then travel, as they say, broadens the mind, and may lead him to discovering what he'd like to do. If he also had a goal of saving towards this it may be more of an incentive to find more work.
Found old thread re kids handing up money at home here:
http://www.askaboutmoney.com/showthread.php?t=14691