My rabbit keeps getting a slight cold and sneezing?

Hi Yuno, my 7-month-old boar once had bronchitis and an inflammation of the nasal mucosa. This was treated and after a follow-up examination he was healthy again, apart from the fact that his throat was still a bit dry. However, he keeps getting a runny nose and sneezing, and it isn't getting any worse. If I give him saline solution or camomile once or twice a day, it goes away for a few hours and according to the vet he is healthy again. My question is what could be wrong with him, since it has been going on for two weeks. It doesn't seem to be contagious, as my other furry friends are still healthy. Could it be an allergy? Another thing to bear in mind is that he is an ermine and an albino, so I would be grateful for any answer, no matter how small. ^^

That's what it sometimes looks like with the nose.