This is what has worked for me, in the past, with my children. First of all, one of the reasons a child refuses to do something, is usually to attempt to exert or gain some sort of control for themselves. One of our jobs as parents is to teach children to make choices, and become more and more independent, while at the same time, keeping them safe and doling out responsibilities that are appropriate for their age. I have found that one way of being able to do this is to try using a timer. Set a timer for and give your son a warning, and tell him that when the timer goes off, it will be time for him to get into the bath. Tell him that if he refuses to get into the bath when the timer goes off he will lose a privilege for that night (whether a tv show he likes watching, or a desert with dinner, or a computer game, etc). Explain all of this ahead of time. Use this method, in conjunction with a visual schedule. I love using a picture schedule. I take pictures of different things that my children need to do during the day and make a magnetic (or velcro) chart that they can help me set up in the morning and as they finish each task they get to take off the picture and then earn a bingo chip (or marble, or pebble) that at the end of the week they can turn in for a prize (like an hour of my undivided attention playing a board game with them, or an ice cream or treat at dunkin donuts, etc).
what is the worst thing that could happen if he doesn't have a bath that night. he smells? Part of parenting is picking your battles and unless its a matter of safety, some battles are not worth picking. However, a bath still needs to happen and that is where the loss of privilege comes in. Also, if this is used in conjunction with the picture chart that he is also earning chips for (or stars or checkmarks) then when he sees that he hasn't earned enough stars for his prize he will be disappointed and try harder next time. Start with smaller, more immediate increments. At the end of each day add up how many stars he earned and let him pick a prize for that night (an extra book read to him at bedtime, or an extra half hour of tv time, etc). Gradually, after a couple of months, you could lengthen to weekly with bigger prizes. Does this make sense? We all, as adults, get paid for our responsibilities in a job. By having your child earn stars (which as he gets older can transfer into an allowance) you are also teaching him about working.
I hope that my suggestions were helpful to you and that i answered your question fully. If You are not satisfied with my answer please let me know so that i might help you further.