Hi J., I'm a nurse and a mother so I hope what I say helps a little. I know it's frustrating to think you are out of the "diaper stage" only to backslide to bedwetting, but it's very normal for young children to regress a bit. Boys typically potty train a little later, but if your 3 1/2 year old has already shown the physical ability to stay dry at night then you have to assume the problem is with his motivation. Since you mentioned that you also have a 5 month old it could be that your older child feels a little left out and with 3 yr old logic thinks that if he's a "baby" he'll get a little more attention. I'm not implying that you aren't paying enough attention to him (please don't think that I would presume to do that!!). I just mean that maybe the 3 yr. old is feeling a little upset about having to share "his" mommy? Try scheduling a little "mommy time" with him every night right before bedtime (in addition to whatever bedtime routine you already have in place). Maybe play a game, read a book, or just cuddle and listen to music. Spend about half an hour with him and make it clear to him that this is "his mommy time". If it's possible to have your spouse watch the baby, that is. If not, then let the older child "help" you with the baby and make references to what a good helper he is and how he's such a good big brother. Try to make him feel special for being the "big boy" but don't make him feel like he's being "bad" for wetting the bed. Try not to dwell on the bedwetting thing with him at all and just let him make the decision on his own. He will most likely grow out of it soon enough and, once he is able to understand that his baby brother didn't replace him in your heart, he may begin sleeping through the night without accidents again. If you are very concerned you can contact your doctor to make sure that he doesn't have an infection (rare in boys this age) but it's one thing to rule out if all else fails.
I hope this helps. Good Luck!
R.