Children often suffer from cough and cold predominantly due to viral cause. Providing steam inhalation should help in diluting the cough. Antibiotics are not required. Some antiallergic like levocetirizine syrup may be given for a few days. Ideally the birth weight triples by 10-12 months of age. Although 7 kg is a bit low for a 3 kg birth weight child, I do not think that there is any need to panic. A single weight does not convey much information. You need to take weight at 3 months intervals and plot it on a growth chart. If the weight follows the normal pattern but is on the lower side, you need not be concerned. But in case it does not, follow up nutrients might be required. How much supplementation needs to be done, depends on the situation and you can be discussed only by your treating pediatrician. All you need to do from your side is to give semi solids in mashed form, roughly at 2 hourly intervals. Home made semi solids made from boiling rice and pulses along with vegetables and giving it after mashing it, should be fine to help your child in gaining weight.