Do you believe in the phrase "a man is know by the company he keeps"?

That's a quote by Aesop, the chap who wrote the fables: "A man is known by the company he keeps"

Yes there's a saying here in Ireland
"show me your friends and it will show me who you are"
Or something like that

Just cuz his friends are liars doesn't mean he is.

Yes. I agree . It not only applies to men. It applies to everyone

There is a proverb in the bible that goes something like "bad company corrupts good character".

Then again I am not one to follow the "sayings" of others, but rather follow my own instinct and learn from my own experiences.

So the decision is yours to make about how you want to treat the situation.

Yes, only what if he doesn't keep any company?

Hmm those are not good signs, but I wouldn't doubt him unless he gives you an actual reason to or unless you catch him lying about likewise things. Is he easily influenced by them?
Friends don't necessarily have the same thoughts about how they should treat their s.o.

On some level yes.

I think many people your age cheat. They don't know better. You know your bf best. What do you think? I doubt immorality rubs off on a person. Liars are undesirable. How can they make him lie?

I'm not a big fan of guilty by association but sometimes it pays to at least be aware of someone's habits and their company.

Yeah I do believe that quote.

We are about the company we keep.