Well...what *I* would do is start taking him to the bathroom about halfway through his sleep time...So, if he goes to bed at 9pm then get him up at about 1 or 2 am to potty. Do this for awhile and see if that helps.
As to the hiding it....that is perfectly normal...he is embarassed and ashamed. Try not to make him feel bad about it...but explain to him that these things happen and it is best to deal with them immediately...and not wait until morning. But, hopefully, the above plan will help to eliminate the problem. Good luck.