Don't worry la. At least your B grow more than double la. As long as he's healthy, that will be ok la.
My B also born in 34th week. I start to bed-rest around 28 weeks. Really worry a lot and always crying at that time because doctor told me he's only 3lbs. Luckily he has 6.5lbs at birth but still need to入箱仔 for 2 days. He always 嘔奶 during 2-3 mths. Doctor give him Infacol, and that's very effective. Now he is 4.5mths and has 15lbs la.
I also give him some 糊仔 once a week. He like it very much. But if your B still 嘔奶, you may only give him liquid food at a later stage.