Most cheats when caught only admit what they think they've been rumbled for.
If he knows all you know about it the flowers, that's all he'll admit to.
It can take ages for it all to come out
e.g.
"I fancied here but I only sent her flowers, nothing else"
"Oh OK, actually, you're right, I did kiss her, like it says on the message you just found, but nothing else happened, it was just the once"
"OK, it has been going on for months, but I didn't have sex with her I promise"
"Yes, you've found evidence of us staying in a hotel, but we never had sex, she took all her clothes of but I thought of you and I felt too guilty to do it". (My ex said this one - OW was pregnant by this time. Heard of other men saying it here also.)
"Oh OK, yes we did have sex".
To learn all this drip drip over weeks months or even years is awful.
Make him show you the messages tonight (or better still get the courage to look yourself, change his password).
Don't accept no for an answer.