I would focus on high-calorie foods and if you're worried about vitamins and minerals do a multivitamin. I got DD eating more foods by letting her take bites of everything we're eating, except the things she doesn't have enough teeth to handle (she only has 6, so there are things like lettuce that she just can't chew up), and she actually climed on the growth chart between 12 and 15 months, though she's still below the 5th, but since she went from less than 14lbs at 9 months to almost 18lbs at 15 months, her ped has been quite pleased.
I'm also wondering how much whole milk he's drinking? could it be that he's mostly filling up on milk, and then not taking solids? Milk is not the miracle food the USDA claims it is, and if he's still nursing, there's really no reason he should need it. DD's favorite foods right now are burritos, yogurt, bread, peas, and this week she wants to carry an apple around all day. One thing I've noticed is there are some foods she'll like one day and refuse the next, so I've taken to offering at least two foods at a time so she has a choice, because if I just offer something she doesn't want she'll give up on eating.