Well I wouldn't do it- but that's nothing to base your opinion on
The food you have him on now is a great food- the prescription- not so much.
Does the vet think it is a food related allergy?
is the food he is on now chicken based? A lot of dogs (mine included) are allergic to chicken. Layla is so bad that within an hour of giving her some she is chewing her paws and scratching at her face. Before going to the prescription I would switch food- Something like Natural Balance has rare protein sources like venison, duck, fish. The "rarer" the protein the less likely your dog will have an allergy.
ALSO (sorry- we know a lot about allergies!) this could be an environemental issue. If that is the case- the diet change will do you no good. I would not spend the money on the food until you know if it is diet or environmental.
I would change to a rare meat- give it a few months before you decide if it is the food- you will not see an immediate change. Keep him on a high quality food- i think it will be better for him in the long run.
If he is having a particularly bad day you can give him Benedryl. Layla is 6 lbs and she gets 1/4 of an adult regular strength (25 mg) pill every 3-4 hours.
Hope that helped.