Jilly,
When this happens, they are usually either expereinceing a sudden fear or anxiety or they have developed a dislike for the sensory experience. Which do you think this is? That will change your approach.
If it is sensory, the toothpaste may be texture, or it may just be the feel of the bristles. If it is anxiety, he might night like the "invasion" of anything, and his fear is going to be inexplicable and you probably will not be able to reason with him to make the fear less, even if you can figure out what it is.
You can search for different toothpastes and try different brushes if you think it is a sensory thing. If it is fear, you have to decide how long you can go without, and either wait for him to be OK with it, or go ahead and do what needs to be done and comfort him afterward. You really can't leave him dirty without concequences, especially with teeth, so avoiding tears is probably not in your future.
The bath is the same. If he is afraid of the water, you will just have to wash him quickly and get it over with. If it is sensory, try a visor to keep water out of his eyes, and other products that change the way it feels.
If it continues, and if he seems to have other sensory issues pop up, you can seek out the advice of an OT who can offer some desensitising exercises, like brushing, but you will not want to try any of these on your own.
See if you can tell what the origin is.
M.