I mean find his actual voice. Pick an actor/random person who sounds like him and whenever you write from the character's POV, "listen" to actor/random person say what you write. It helps, trust me.
2. Know your character.
I know, this is a pretty boring tip that's pretty typical in a post like this. I get it. But it does help. If anything, it's a stepping stone to the next tip.
3. When you write, "get into character".
People who act will know what this means. If you aren't an actor, it means to get into the mindset of the character. Become him, in a sense. My acting teacher once told me that when I'm acting, 90% of my brain should be character's brain, and the other 10% should be my "actor brain" (or writer brain, in this case). When you write, you want to become the POV character. The previous 3 tips should help with this.
4. Myers-Briggs Personality typing.
This can actually be a huge help. Simply looking at the different personality types and deciding which one you want the character to be can help separate him from you. One pretty helpful thing is that it can help you decide what the character would do if in ________ situation.
5. Make the character be opposite your gender.
For some strange reason, I like writing from a guy's POV better than a girl's POV. I think it's because it creates distance between the character and myself. I put this as the last tip because it isn't always possible. I mean, who would want to read a book with no girls?