This is an area I have been asked a lot about and have put together a fact sheet on it, which is now on my web site (www.sneakys.com.au
under the resources section). When someone says `underweight` it shouldn`t be confused with `not on the 50th percentile`. though I can totally understand your concern if your little one isn`t on the charts. If your toddler is healthy and growing well then even skinny kids are likely to be just fine. Like adults they vary in size and shape. To some extent you may be fighting genetics here, if you or your partner are tall and slim then your toddler may just be heading to his genetically predetermined size. If your little one has grown at a consistent rate without any sudden or sliding movements down the percentiles this is more support that things are okay. Still the trick to weight gain (healthy weight gain) is protein and yes calories.
Medically most professionals use a high calorie powder (pediasure), personally I look down the protein powder lines (ones specifically made for children such as Kids MetaMeal by Metagenics in Qld) and a protein balanced diet. You may need to see a professional for a script for the powders which ever way you go, I like the kids protein powders because the good ones are a balanced meal support and generally very low reactive, rather than just focusing on fat to increase calories.
Keeping allergies in mind you could look at adding some ground nuts and seeds (such as sunflower seeds, almonds, linseeds, pumpkin seeds etc) into his meals each day, this will increase fat and protein and essential fatty acids as well as loads of other nutrients. You can buy them in packets yourself and then using a coffee grinder, grind them to a powder and mix together.
Smoothies are fantastic, use your choice of milk, add a nice yoghurt and some fruit, they are also great as ice-blocks in summer. You can pack a lot into those, even your seed meal! Eggs in various ways, scrambled, hard boiled etc are also great. Dairy is a great source of protein too. You could also try mashed potato with cheese and a knob of butter, toppings on crackers such as cream cheese or almond paste (I think a better option than peanut butter or other nut spreads). Other protein foods include fish, meat, nuts, beans, seeds, legumes etc. In terms of quantity, about his palms worth in each main meal and if you can in snacks too.
Also offer small portions of food regularly over the day so as not to overwhelm them with a big meal, toddlers have small tummies, plus finishing a plate off is a good motivator for your both.
So I hope this and the tip sheet help and I hope things improve for you shortly.
All the best,