I had this with DH. Shortly after we started seeing each other I realised he couldn't make decisions or take initiative for anything. After a bit more time, when I realised I wanted to be with him long-term I started to think about how to try and improve things, as I didn't want to be in a relationship where I was always in the driving seat.
I realised a lot of his trouble came down to issues from his childhood, so started off asking him to make decisions about really simple things; eg choosing first when we went out to eat, then asking him about why he'd chosen that, saying it sounded great etc. So we started small and worked from there. Like Duffedup says, offering loads of positive affirmation nd praise etc.
This definitely helped his confidence and, after we started living together, I asked him to take responsibility for certain jobs/ areas of our life, that he needed to deal with, eg car was completely his responsibility. So he had things to "be in charge of". That helped too.
It's important to say that this wasn't just a learning process for him stepping up, but also for me, in stepping back. Im quite a bossy, mouthy cow strong-minded and assertive person and I had to resist the urge to take control and instead to let him flourish in his own way.
A few years on and he's a fantastic, supportive, confident husband. We have a great relationship, because we've learnt what each other can bring to the party and how to let our very different personalities compliment each other. It took work from both of us to get there though.