Hi there. I can totally sympathise with you as I had exactly the same problem with my daughter who has iv. She is now 10 so be assured you will get through it. I used to have to swaddle her until she got big enough to get out of it and then I done exactly the same as you do now...
I am in the uk so if your not then I suppose the medication would be different. She used to bathe daily in a bath oil called balneum plus. It is an oil that contains an anti puritic which acts as a slight anasthetic. Used just before bedtime may help.
Also check the age range on piriton...dont know if he would be old enough to use it.
Good luck and stay in touch. Even if you just wanna chat.