We're really glad you took the time to reach out for some help with your parenting question. Having a young child who is biting consistently must be very frustrating for you to experience, any parent knows how upsetting it can be when a child is engaging in chronic misbehavior such as biting.
Does your son's biting seem to be a way to express negative emotions? If so, this offers you a great teaching opportunity. At 18 months your son has the ability to begin making simple associations between behavior and consequence. When your son bites provide him with a consequence, such as placing him into a time-out. Explain to him that he must sit there for a period of time quietly and away from fun. Have it be for about 30 seconds or so. Once your son is able to sit quietly remind him why he had to sit there (biting). Then, allow him to go on his way. Your son may initially resist the time-out by screaming, running away from the time-out chair, etc. That is normal. Calmly go after him, set him back into time-out and start the process over again. This can get extremely frustrating, but the last thing that you want to do is give up. Your son can definitely be taught the difference between appropriate and inappropriate behaviors, and this is a GREAT place to start.