my son was like this and is as healthy as can be now, 4 years old, 114cm tall and 3 st exactly. he was born at 8lbs 4oz, went down to 7lbs quite quickly (ex wasn't producing enough milk). he dropped centiles here and there all the time yet after finding soya formula (cows and goats formula would not stay down) he stopped loosing weight and slowly put it back on, an ounze here and there but never loosing any. he still looked small for his age in weight terms but was healthy. never bothered with high calorie foods, as long as he added weight and didn't loose it I was not worried. HV kept saying we had to feed him up, put some weight on him, but studies show that the quicker you put on weight when a baby the more likely you are to be obese later in life, fat storage cells are only ever made and never destroyed by the body, the more you make earlier in life, the more there are to store fat in later in life (and therefore the more cells the body has to fill up).
my tutor at uni always said that if a baby is putting on weight, however small the gain, it still means that baby is getting more calories than it needs, therefore it adds weight, if it were not getting enough, it would loose weight, if it stood still, then it was only getting enough to sustain itself so was 'living on the line'.
avocado is rich in fat so beware of that, also beware of sugar/starch rich foods as they release energy very quickly and are more likely to end up stored as fat. complex carbs and proteins are best, they release energy slowly, giving the body a chance to use it rather than store it.
www.milupa-aptamil.co.uk/en/static.asp?mode=Pregnancy_landing&utm_source=Google&utm_medium=PPC
www.cowandgate.co.uk/feeding_and_nutrition
a couple of sites I found that may be of use