Despite what most pot smokers say, yes it is addictive and habit forming. Habitually if not chemically. I don't know many to have stopped completely before until it begins interfering with their jobs and families.
If he's addicted, it's hard to stop. Be glad he told you he's willing to quit for you. Try to believe him, you should trust your boyfriend. There's no thing as Pinokkio's nose to see if someone is lying, but why wouldn't you trust him? Give him the benefit of the doubt, it's quite good of him to (try to) stop because you don't like it.