Taking Piriton tablets for your cat is fine, however, giving him Piriton syrup containing ethanol is not recommended. Cats are susceptible to ethanol toxicity, so you shouldn't give any product containing ethanol to your cat. You should stick with the tablets as they are easier and safer to administer than the Syrup. Furthermore, The taste of the Syrup would make administering it to your cat very difficult and probably cause your cat to drool and/or foam at the mouth, so stick with the tablet.
Usually, 2 mg of medication is given to a cat every 8 to 12 hours, but alternate forms like transdermal or other medications are preferred in cases where cats are unsatisfied with the taste of the medication.
*This information is not intended in any way as medical advice. Before giving any medications to your cat, you should consult your veterinarian.