it is really common and normal for babies to swap percentiles after the birth. In utero a baby's weight reflects the mothers nutrition and effectiveness of the placenta. After birth they move towards their genetic potential and are affected by factors in the environment (ie their diet etc) The fact that he has dropped compared to his original percentile doesn't mean he is *underweight* (anymore than if he increased his weight in comparison to that original percentile would mean he was *overweight*) In the first 3 mnths it would be normal for babies to put on 150 g per week, and then over 3 mnths you would expect weight gains in the order of 70-100g per week.
Often family history can give you a really good starting point. Are their people in either side of your families who have intolerances or allergies? Eczema? Asthma? Coeliacs? Are you and your partner big people/small/average?