It may just be a phase, but it can easily become a habit. I think 6 month olds should still be having 2 sleeps.
MAybe try putting him down before 10 so he's not overtired and harder to get to sleep. I know its really hard to listen to them cry, but they can be crafty little things and learn really quickly that mum will come pick me up. Personally, I would just leave him to cry and ove a few days he should relearn about day sleeps. You could do it more gently than that tho, start with waiting 5-10 mins then instead of picking him up just give him a pat or soemthing.
Good luck