I have been a parent and am now a grandparent of 14 grandchildren.  My daughter-in-law had my grand-daughter (age 3 1/2) deal with her own messes.  If she messed herself... she had to clean herself up!  When she came to my house I was always there to assist her.  What I discovered?... She took responsibility... more so when she had to correct the problem herself!  Her mom sent her to do it by herself.  I did it a bit differently but I feel I got the same results!  I accompanied her but was only there to help her figure it out and assist where I could see it was completely becoming frustrating for her to do.  I assisted in a teaching manner but also helped her to prevent a bigger mess (I also cleaned her after she had cleaned herself... I just can't get past being a nurse!)

I agree with the expert to place a waterproof mattress pad over the mattress to protect it from damages, but I disagree in having a 3 1/2 year old child strip the bed and change the sheets.  That's a little bit of a hard task for a 3 1/2 year old to take on by themselves.  I know it's difficult not to loose your patience, especially being a man (no offense) but my husband also ran a little thin in the patience area when it came to the potty training.  

My advice-
1.  Don't give him too much liquids to drink as it gets close to bedtime.  If he's thirsty, just a tiny sip of water.

2.  Make sure that he uses the potty last thing before bed.  As soon as the teeth are brushed, use the toilet, wash up and then straight to bed, read a story to leave him at ease.

3.  Even at best, a child that young will have the occasional accident, don't punish the child.  It is not too much to have the child clean themselves and change their training pants though, they can handle this.

4.  On the nights that he does not have an accident, reward him.  He'll soon see that it's better not to have the "accident".

It probably is an attention issue, good or bad attention, they have your attention, maybe by reading the story he'll get enough attention to settle down and rest.  Try to be patient, each child develops at a different pace and they go through different phases, this one will pass too.

If he continues for a long time to have difficulties with this matter, please discuss it with his pediatrician, because some medical conditions do cause this to happen and his doctor should be made aware of it.  Best wishes!

Thank you everyone for your help......I did use the reward method which started right after the expert advice was given and I thought about having him change and clean himself, but thought perhaps too harsh. I asked my sister and she said just be patient and like the last person said - he is seeking attention. I am happy to report that it was only that one time, but as you could tell I was rather at my whits end and needed some advice and help from the outside. Again, thank you and please keep the advice coming, I really appreciate all I can get.