Is your vet prescribing this to give as needed?
I read over your other threads in relation to Duskys sneezing. I see you’ve already tried using alternative hay. Oat, wheat, barley or orchard hay are some others you might like to try. There are some companies that do dust extracted hay that might help. ( http://www.dustfreehay.co.uk )
I’ve also heard of people misting the hay or dipping it in water. Wet hay though must be replaced daily as its at risk of getting mouldy. If you were to try this, I’d only do a small amount at a time and see if your rabbit is bothered by it. Some might turn their nose up and not eat the hay at all.
Do you have the hay on top of litter material or in a rack over it? What type of litter do you use?
This question might seem odd but is where he eats his hay near a heating source? Warmer area of the home?
This is another thing I have read sets off sneezing fits funnily. One owner reported their rabbit with chronic nasal issues needed the cooler air to keep their respiratory passages open. They still housed them indoors but had a window open in the room the bunnies were set up in.
I’m sorry I haven’t addressed the question about piriton. I’m not familiar with it. I have heard of Benadryl been given for same issue.