Hiya Lily ,
He is a beaut and i am well into my dogs , so i'm goin to tread carefull now and try to not offend you in things that might sound a litle offensive , they arnet intended to be though honestly .
I see he is only 9 months old and you say great with the ex , and that would be because he see's the ex as master, now the ex has gone you need to become pack leader, it will benefit in all areas not just on the lead , a dog that leads you and walks in front of you is't respecting you.
So it starts in the home ie dog only eating after you, so you pretend to eat first, remove his food half way through , then ask him to sit and give it back. Not let the dog on your seats , upstairs etc , i know its hard , my last dog was a big boy and he just wouldnt get off the sofas after we let him as a pup so we had to gate off the living room and he pulled like a bitch lol but my OH let him and i didnt so we were inconsistent too.
The dog walker is a good idea , its WAYYYY too hard with a pulling dog and a pushchair x xx