One of my children was a bit underweight and really didn't want to try new foods after being weened.  

We saw a nutritionist and sought advice from friends, and here's some tips which worked for us.

1) Talk to your doctor / nutritionist

Get actual advice and measurement and a plan from them, not just this blog :) 

2) It's ok for your toddler to eat the same food a lot

In our case, that meant sausage every day, some times twice a day.  Seemed horrible to us, but kids don't mind repetition in diet.  And calories are calories and protein is protein.

3) Smoothies were a big win


We bought a fancy blender, frozen fruits, and made a LOT of smoothies.  Almost daily.  And when we watched movies, we called it "Smoothie and a Movie" and he had to drink his entire smoothie before popcorn.

4) Carry Food With You and Offer Constantly

We called this the "Bacon in your purse" diet.  It involves making sure that foods your child likes are not just offered at mealtimes, but throughout the day.