Is It Safe To Skip Last Dose Of Anti Rabies Vaccine?

my son was bitten by our dog with anti rabies a month ago only we decided to inject him an anti rabies even our dog has already. supposedly the doctor said he must have 4 doses of vaccine. Unfortunately on the forth dose he wasnt able to have the vaccine because he got sick and must take a medicine for that. my question is .....is it ok not to have the 4th dose since its already 2 months after the bite and the dog is in good condition?