Does this 'friend' (yes I use the term loosely) deserve to know I'm seeing his ex?

Yes he does, how would u feel if the situation was reversed. If u don't say anything now when he does find out he will probably think u always wanted her or was seeing her behind his back. Ur not asking his permission but getting sum unneccessary drama out of the way in case he sees u 2 together or here about it from someone else.