An enema for a 6 week old? I think you can try other things first. Even a suppository is a better last resort thing to try. First of all, how do you know he's constipated? Is he passing hard stools when he does go? Breastfed babies don't get constipated as there's a natural laxative in the breastmilk (hence the loud explosive runny bowel movements!). What this could be a sign of, which is more of a concern, is that baby is not getting enough milk--his body isn't producing much waste because it's using up everything.
If he's bottle fed, then this page might have some good ideas for you, including putting him up to his belly in a warm bath (someone will have to hold him up, of course) and massaging his belly and bringing his knees up to his chest.
http://www.askdrsears.com/html/8/T081100.asp
When our breastfed baby wasn't pooping for a few days, I used a q-tip dipped in olive oil and very gently massaged his anus (sticking it in only a tiny tiny bit--like a millemetre) and massaged his belly, and soon it all came out! But he also had issues with not getting enough food at first (my milk took a few days to come in and I didn't have much colostrum).
Here's another example of what this could be:
http://parenting.ivillage.com/newborn/nbreastfeed/0,,3wxq,00.html
Good luck!