Start from the ground. Your presence will be of comfort to him. You might need to walk in front of him to show him it's okay and then you can ask him to cross by himself.
When you ride, he is alone so he will need to have enough confidence in himself before you attempt to ride if you want better results.
I also think that you can also prepare your horse for an encounter with water by asking him to walk over unusual objects, such as sturdy plywood panels, or a tarp. This is a great simulation for crossing water because it helps him feel more confident in general about putting his feet on something other than solid ground. When he willingly walks over these without fear or hesitation, it should be easier to convince him that water is safe, too :)
Start at Home! It goes without saying that your horse’s first introduction to water should take place in the safe and reassuring environment of home. Before you ever attempt to cross water on the trail, your horse should be calm when you bathe him. If he hates getting his feet wet and won’t stand in a puddle when you give him a bath, you can hardly expect him to forge through water on the trail. Spend time working with him so he is completely comfortable with water at home. Let him sniff and drink from the hose if he shows an inclination.