Please don't tell him OP.
I was in a very similar situation with my DS. He once ran up a £300 phone bill and came out with a load of bullshit excuses - someone had snuck into his bedroom and dialled, his phone must have fallen on the floor and dialled an international number, blah blah. I had sight of his messages where he'd said to the person he'd called "whoops didn't mean to talk this long, bet my mum will bollocks me, oh well lol" 😠
I gave him chance after chance to tell me the truth but he just wouldn't. I lost my temper and snapped "You're a bloody liar just like your dad."
It badly damaged our relationship for a long time and affected his self esteem and security (as he was thinking - she left my dad, I'm like my dad, she'll leave me too)
Worst of all it didn't even stop him lying to me. So I wounded us both for no reason.
If I could wind the clock back, I would have told him that I knew he was lying, and explained how lying had eroded trust and made me feel very upset, disrespected and disappointed.
Hang in there. the teen years don't go on forever, and we do have a great relationship now he's in his 20s.