Well, I think the first thing a lot of parents do that make is harder to wean is not giving their babies sippy cups at a younger age. For instance both my kids were on sippy cups at 5 months. Also, I am a childcare provider and if I got a child as an infant, they were the same as my children (So I know most 5 month olds are capable of this). When the ped. said they could have water I got a sippy cup.At first you will have to hold the cup for them, but soon they will be doing it on their own. I never, ever put anything in a bottle besides formula or breast milk. I also give the milk in a cup at meal times so the baby gets used to getting milk in a cup as well as the bottle. This REALLY helps with weaning.
Also, I never allow a child off my lap with a bottle. I ALWAYS hold babies while they get a bottle. When they get older and are moving around a lot they usually do not want to sit on your lap with a bottle anymore. I allow them to walk around with the cup, so this also helps with the weaning.
I would just offer him a cup as often as possible. Give it to him with all his meals. Try a cup with no lid at the table. Some children prefer this, and most babies can start to try liddless cups at 10 months. (according to the Montessori way of teaching children should use only regular cups by a year old!) My 9 month old loves drinking from liddless cups! She makes a mess, but it makes her very proud of herself. Also- try the sippy cups that are NOT spill proof. These seem to discourage children at first. They do not understand how to use the cup, so if nothing comes out it fustrates them. I REALLY like the Toss and Keep cups. They need immidiate results. They have the ones with straws a lot of children seem to enjoy as well. Once they have these cups down pat you can try the spill proof ones.
Also, do NOT worry about taking away the bottle just yet. Let him get used to the cup while also having his bottle. You dont want to give him something new and take something away at the same time.
GOOD LUCK!