Can a decreased level of testosterone in a male cause weight gain?

Maybe but it depends: How decreased is decreased? And decreased relative to what? You're actually asking a very complicated question that medicine is trying to answer. For instance, we know that hypogonadism (truly low t) is associated w/weight gain. But we haven't proven cause & effect. Perhaps, it's the weight gain that causes the drop in t. But when we give hypogonadal men some t, they feel better, then exercise ...