My 23 month old was born premature so he is small for his age, weighing only 22 lbs. Our challenge with him is to keep him on a nice, steady growth curve and keep him gaining weight. A nutritionist sees him at his regular pediatrician visits. The first thing I would say to you is that if your dr is comfortable with your son's weight and feels that his growth rate is normal, I wouldn't worry about it too much. If you are still worried, go to another dr for a 2nd opinion or ask your current dr for a recommendation to a nutritionist.
It's hard to get toddlers to eat well. The key is to make smart choices and slip extra calories in wherever you get the chance. Small amounts can provide big calorie payoffs! I've got a great handout from our son's nutritionist that lists calorie boosters and snack ideas.
We don't have our son on a multivitamin, but with the recommendation of our dr, we supplement with Pediasure drink or Carnation Instant Breakfast (remember that stuff)? The Carnation is a great because it has lots of vitamins & minerals and if you The handout is 3 pages long, but if you'd like a copy, feel free to send a private message to me with a fax # or I may be able to scan it and email it to you. Good luck!