It would say, it depends on a few things. Does he have children with his wife whom he is supporting? Does giving him a place to live at $100 help ensure that he can make support payments? Is he working a full time job? Does he use the whole house including the common areas, vs. the bedroom/bathroom/laundry, plus storage (basement? attic?). In Boston, it costs $800-$1100 for a room in a condo or apartment with access to laundry and shower and kitchen.
I think your husband needs to sit down with him and ask what his plans are to "save up and move out" - that's very vague. If he's banking almost his entire salary less the $100 he pays you, that's one thing. But if he's going out, eating in restaurants (since he's not eating much with you or apparently even buying his own groceries), and so on, then he's spending all that disposable income on partying and fun, and he's taking advantage of you. So I would say, since he's been there since February, you need a rental agreement (you can find "boilerplate" samples on line) with a higher rent which also spells out his privileges and access, or you need to set a departure date. If he is storing his boxes in his room, that's one thing, but if he's spilling over into your common space (garage, attic, whatever), then he needs to find a storage place or an apartment.
But of course, your biggest problem is your husband, who does not think this is a problem.