I had this with DD2 as well. It took me from 3 mths to 5 mths to get her to take it. Try at different times of the day - night time might be when he wants comfort.
Also try mid-feed or when he's not particularly hungry so he's not frustrated.
I had success eventually with Nuk latex teats - the brown kind, rather than the clear silicone teats. They're much, much softer than silicone and thinner too, so more like skin, iyswim. As DD2 got a bit bigger, the latex teat then started collapsing as she was sucking too hard, but she was happy to take the Tommy Tippee Closer to Nature. The Nuk teats were great just to get her used to taking milk from something other than a breast. Incidentally, I have a MN friend who said a couple of her babies would only take latex at first.
Also, a breastfed baby is often used to quite a fast flow, so don't get on with the slow flow teats.
It does sometimes take perseverance. I tried all sorts of things, like hiding the bottle under my arm while my boob was out! Eventually she just realised I wasn't trying to poison her and got on with it.
A word of warning though; when he does take it, don't leave it too long between bottles. That is, not more than a day or two, because you don't want to have to go through it all again!
Good luck.