desmo tabs

is anyone elses child taking desmo tabs or desmo melt for bed wetting,my sons 8 and has never been dry at night,he started taking desmo melt last year,it worked for a while but then stopped,so i continued to change his sheets every night,then the doctor prescribed desmo tabs,which are working for the time being,we tried for 2 weeks without them last month and he went back to wetting,the hospital say it's because he isn't producing the chemical that tells the brain not to wet at night,it's quite common and he will produce it in time,but how long is in time and how long will he have to carry on taking the pills,coz the hospital basically said they don't know