What Causes Swelling And Itching In Eyes After Taking Allegra?

My 2 1/2 year old son takes Allegra now for about 2 days I switched him from zyrtec cuz it seemed like it wasn t helping him the Dr also said to get him some allergy drops because his eyes swell up and itch to where he s constantly rubbing them. Anyway I bought him visine-a for allergies I just looked at the bottle and it says for 6 years of age and older so im wondering it s ok to keep giving him these drops he absolutely hates me putting them in but his eyes are so bad.