Use ghillie armour to feed him. Dont walk too close to them or they will hit you and you will lose your taming progress. Just give them space. Another good way to tame them is to make them mad and lock them up in a cage 1x1 with a hatch as roof then get him in there and get in the roof and feed him!
It literally only took me an hour to tame a level 145 with only 50 mejoberries.
1. Gather berries and put the berries in your last slot
2. Kneel down and walk up to one of them and feed it the berries
3. Now you just follow it around while kneeling down and feeding it berries whenever it's hungry
4. Congratulations you tamed one.
Since they're commonly found in the red wood forest, I recommend only taming one if you're close to water as if you get attacked by something you can jump into the water to get away. Then when you think it's safe just go find your monkey and go back to following/feeding it.