My guinea pig is about 3 years old and is an American guinea pig

Hi! My guinea pig is about 3 years old and is an American guinea pig. He’s had respiratory problems for around a year and a half. We’ve taken him to the vet about 4 times and tried antibiotics several times, but they didn’t really help. He still eats and drinks really well, poops normally, and walks around, but his breathing is almost always noisy. Sometimes he lifts his head and seems to struggle to breathe for about 20–30 seconds, then sneezes really loudly and starts breathing better again. He doesn’t breathe through his mouth and never squeaks in pain. Recently, the noisy breathing, sneezing, and these episodes have gotten much more frequent, which is really worrying me. Unfortunately, my family can’t afford expensive tests like X-rays or surgery right now. Is there anything affordable and safe we can do to help him and make his breathing easier? How uncomfortable do you think he is? And do you think we should take him to the vet again soon? Thank you so much!