I have posted a similar thread (nearly 4 months with no weight gain). Bradyn was stuck at 8.5 kg from 6 months.
I saw my clinic nurse about this. I know there are some differing opinions around as some nurses err on the side of caution. My CN said if he is not throwing up after meals (can indicate a urinary tract infection) if he is still happy and developing and growing and if his weight has not crossed two lines on the growth chart, then things are still ok. I was advised to try to add an extra bottle during the day and make sure to fill him up at meal times. You can't over feed or force feed a baby they will only eat what they can, so you can also try a few snacks in between meals too, if you aren't already.
The week of visiting my CN Bradyn did start drinking more milk anyway, I made an appointment for another weigh in in three weeks from seeing the CN for the first time. He now weighs 9 kg (at 10 1/2 months) and we will check his weight again when he is 12 months.
Goodluck, I hope your boy turns out like mine and will start to eat/drink more. As long as he is not doing the things mentioned above then I am sure he is fine, at least for the moment. Best to keep monitoring his weight and if you have any concerns see the doctor or nurse. At least you know your not alone.
Jeninne, WA, Bradyn 24/11/04, #2 Ryan 01/12/06