Let me tell you what else he is doing while ringing you and telling you he's low.
He'll be having a separate little drama with the OW, my wife found out, she kicked me out, poor me, I still love you, I won't be getting back with her, when can we meet again etc etc
Don't let him back in (certainly not yet). Pack whatever stuff he might need, have it sitting outside for him on Friday, direct him to the nearest laundrette, he can go back and stay at his mother's.
Are you sure he's at his mother's? It would not surprise me that he has been staying at OW's (my wife has kicked me out, it's only temporary, I will sort it out etc etc)
And I agree with anyone who has said, don't engage esp not in the phone - ignore the calls, ignore the texts (put a horrible ring tone on his calls like "duck") so you don't have to keep looking when phone rings.
Only text non-emotional stuff like "packed your stuff, will leave them outside for you Friday" (and find yourself something to do all day Friday so you're not home - massage, trip to see friend in another city)
Oh and a good website which I think Wellwobbly recommended - Chumplady.com