My 6 month old baby has been nose congested for 3 months now & snores at night, tried humidifier, saline drops, is it allegies? Can he take decongestant?

Not the best. Over the counter cold medicines are no longer recommended for children, because studies have not shown that they really help, and they often cause side effects. You should see your child's doctor to figure this out since it has lasted so long. It would be unusual to have this much trouble with allergies so young but there is a simple ige blood test to check for allergies.
Child snoring. Have your child looked at by an ent. It could be adnoids and tonsils that are enlarged causing this problem.