Just to let you know, we just had the Well Child nurse here today, and she weighed DD, who with clothes weighed 11.92 kg (26.2 lbs). She's now halfway between the 15th and 50th percentiles
At 15 months, as I've just checked in her baby book, she was 8.53 kg (18.7 lbs) and was just below the 15th percentile. And by then NOBODY was worried about her anymore, she was obviously growing, slowly and consistently at the bottom of her chart.
As a fellow mother of a small child (and one that WAS
at one point diagnosed as FTT), from your description (and unless you haven't told us that your son used to be 50th percentile and has since dropped down
to around the 15th or slightly below), I can't see how your child is FTT.
As for him not walking yet or other motor development issues, check out this page at WHO:http://www.who.int/childgrowth/stand.../en/index.html
Download "Percentile and means in days and months". I find that so useful when people indicate your child is late at something. Only 50% of healthy children walk by 12 months. I think that at least gives some perspective.
For what it is worth, DD ate much less at that age (not counting breastmilk). And she eats great now, only thing the nurse could worry about was that DD's currently refusing beans, but she said not to worry anyway, that it would sort itself out.
I do think, like pp, that your son will know that you are trying to trick him into eating more than he wants, and that you are worried about food. And it is likely to be stressing him to. He may very well react in the only way he can to gain control: By refusing to eat.
Point is, I think your son is probably gaining just as well as he should, and eating what he needs, and what you really need is to stop stressing about it. Try to make mealtimes fun times instead. Model good eating habits, show him you enjoy food, sit down together as a family, talk about pleasant things, listen to classical music or something like it, light candles, set the table together... Make food enjoyable. He might enjoy helping you prepare a meal (in any way he can). If you can get him to think food is great and mealtimes are FUN, most of the battle is won. And it won't be a battle then, anyway.