State Specific Question About: COLORADO (CO)

Saga continues...Thank you for your patience and any help you can offer.

Tenant gave notice 4 months into year lease, no reason:

So I went to the house and handed him the letter, in more assertive language, that I'd been trying to give to him and his "old" GF, who'd previously moved out without notice. Along with the letter was an Agreement to Cancel Lease form. The letter basically said he can move out, but he's in violation and recommend he sign the agreement (modeled after Mr.LL form), and that, without a Cancel Agreement, he's still under the lease, paying rent and utilities and other obligations until I find a qualified tenant. THEN I finally find out why he's moving--says I'M in violation! Said his ex-girlfriend drove by on the day I said I'd be working outside on window trim, replacing it after house was painted(2 days before he sent me vacate notice and long after she'd moved), and she saw me coming out of the house with a handsaw. Of course, she's mistaken--I didn't enter; I was on the porch, working on installing the exterior trim, door closed. Her word against mine. He's refusing to pay Dec. rent, is taking utilities out of his name and refuses to sign the buy-out(a $500 fee).

1) Does he have to file/win suit against me to be able to withhold rent and cancel the lease for alleged LL violation? (Says his lawyer advised him...hmmm: 'Just move out and quit paying'?)

2) Will I have to file suit against him for rent and utilities? (If I get a renter soon, enough deposit can be applied.) Can I also collect late fees per the lease terms ($25 + $5/day) for as long as he withholds rent? I'm documenting costs to rerent.

3) What kind of notice is given in this situation? (He's moved but I still don't have ex-GF's keys or signature on notice to vacate; he wrote both names and signed alone.)