wait and watch but keep trying
This depends on birth weight
. Normally he should be thrice the birth weight by 1 year if age and now at 18 months may be thrice plus extra 2 kgs.
Your child's weight should be plotted in the growthchart and serially followed
8 kg is low for 18 months .
Hope his milestones are OK.
To increase the weight you should give a high protein high calorie diet.
will not increase weight but rather a supplement when diet is not balanced.
Milk,,pulses( dhals),egg,cheese,etc will give protein.
Juices, little extra oil,butter, will give more calories.
However if the child is otherwise normal and active, give him more time rather than making it a point of tension for both child and parents!!!