Husband never wants sex???

Hi guys and girls!!

My husband and I have been married 1 year - together for 3 in total.

We gave 1 son and I'm currently 6 months pregnant!

He NEVER wants sex! It doesn't matter how often we talk about it or if I send pictures or sexy messages he never seems keen!

I don't believe hes cheating on me... but what could the issue be?!?!