I used the Malem alarm and it worked within 3 weeks - although, I did sleep in the same room as DS when he had it attached, to be sure that he got up and that his body started associating the need to wee with getting out of bed. It was the best thing I ever did.
I used to put him in underpants, then I put the alarm sensor over the top of his underpants, then secured it all with a huge 25kg+ nappy with his pyjamas on top. I always kept a spare pair of undies, pyjamas, and a spare nappy ready to go in the night, so getting up and changing was a quick process. After about a week, he was getting so good at holding it in (once it had started) that we had almost no sheet changes/pyjama changes - just went through the underpants and nappies. After two weeks, the night time wetting almost stopped, but for a few accidents, and after three weeks, it stopped altogether.
Now we have a rare accident - maybe once every couple of months, and it is usually my fault cos I've given him liquids too close to bedtime, or fizzy drinks at any point in the afternoon.
I swear by these alarms - as long as you are willing to help him through the few weeks it will take to get dry.
Good luck!