M.A.
i don't really have any advice, but i just wanted to say that my son is five and going through the same thing. i never thought we would have this problem. my daughter learned to use the potty during the day and night over one month. my son took longer to stay dry during the day but has never been able to stay dry during the night. we too use pull ups they don't do the job. the doctor and my friend with five children told me this is normal, though very frustrating. for over a year i took him to the bathroom about 4 hours after bedtime. however this never prevented the bed wetting. he would use the potty and then still wet the bed. my only suggestions for you, though it has not made a difference for us many have said to try this, is to allow him to drink before bed time. if the body becomes dehydrated the kidneys can work harder to make urine. i have read a few alternative doctors recommend those who wake frequently to urinate drink a few ounes of water before bed to prevent this from happening.
all i can say is good luck. i sympathize with all the sheet washing you must be doing. i keep reminding myself and him that this will end one day- everyone learns to be dry at night.