Sorry, this is long. Short version - DB has been smoking in our shared accommodation and is lying about it.
Long version - Myself, DP, our 3yo and my DB all live together. Property is privately rented. We've been here 5 years and despite all adults being smokers, we've never smoked inside the house. It's against our tenancy agreement, and obviously I don't want my son living in a smokey home. (I know about 3rd hand smoke and we do take measures to try and minimise this).
Anyway, we've recently gone away for a few days leaving DB to his own devices. When we got home we found cigarette ash in the sink (in downstairs toilet). I sent DB a message with photo attached saying 'This is not ok'. He messaged back apologising saying it must have been after he'd been out for a few drinks and rushed in to use the loo with a fag on the go. Still annoyed me a bit, but thought 'hey, these things happen.. I guess!?' Although not when we're home, that I know of 
Next, I notice that a few things on the living room window sill had been knocked over. A little weird. As I'm tidying up I notice cigarette ash on the window sill, some of it on something that belongs to my 3yo. Further investigation, I find over 30 cigarette butts on the ground directly outside the living room window leading me to believe he'd been smoking in the living room as well.
Once I'd calmed down enough, I messaged him: There's also cigarette ash residue on the living room windowsill (back one) including on 3yos belongings and I've just swept up about 30 butts from directly outside the window. I don't really know what to say... did you have some sort of mental breakdown while we were away?! 😑
His reply: No I stood outside there and watched tv through the open window so any "residue" would have been through that.
I wanted to believe that, but there were also marks on the net curtain as though a cigarette had brushed against it in a few places and the marks are on the inside. Also, if he really was outside he would have used the ashtray which we keep out of reach from the 3yo. He would have had to walk past where the ashtray is kept to get back into the house!
I sent back: I'm not as stupid as I look.
His reply: ok whatever
I haven't replied since. When I went upstairs it smelled of stale cigarettes. Then I did something which probably lost me the moral high ground. I went in to DB's room to look for evidence. Cigarette ash on the window sill, empty cans used as ashtrays, cigarette ash in the bin, cigarette butts in the bin. No denying that he has been smoking in there.
So as not to drip feed, there have been a few other issues recently with the living situation. He owes us rent money, he regularly helps himself to our food which we would not mind him having if he asked, or even acknowledged taking it. He lives like a slob, dirty crockery and rubbish all over his room... lots of little things which all add up and this feels like the last straw. We suspect he's been smoking in his room for a while, if not the whole time. We used to find fag butts outside his bedroom window, and every so often I get a whiff of cigarettes, but honestly his room smells so bad that it's hard to tell what exactly you're smelling.. I always put it down to my mind playing tricks on me. I never thought he would be so disrespectful.
AIBU to be angry and upset about this. I think the fact he seems quite happy to lie and make me out to be an idiot has made me feel even worse about it all.
WIBU to go in his room? I feel like he would have just found some lame excuse as to where the smell was coming from if I'd asked him why it smelled of cigarettes upstairs.
We're away again today so won't see him for a few days. I don't know where to go from here. Do I tell him what I found in his room?
Would it be an over reaction to tell DB we can no longer live together. We would all have to move as we couldn't afford to stay where we are without his contribution. I would obviously give DB plenty of notice so he's in a better position to look for somewhere of his own. I feel guilty as the reason we all moved in together was that he was struggling with bills when he lived alone 