Charlotte was about 6 months when we went out for the first time at night and left her with "nanna"
She cried for ages and so we had to go straight home after the movie lol Poor Charlotte was beside herself.
I didnt leave her agian until she was about 8 months old with someone other than DH. She was perfectly fine.

My biggest obstacle to leaving her at night was the fact she was fully breastfed and refused to take a bottle - EBM or formula.

Best of luck! Bugger everyone else - you leave him when and if you feel comfortable and he seems ready