I too have a skinny kid. As long as your baby is following more or less a curve, and it sounds like yours is, it should not matter where they are on it. Ten or fifteen percent points are nothing. My son had not started solids either (until 6m), our pediatrician suggested starting with jarred baby food meats first. Actually she recommends this for most breast feed babies because they start solids later. Then to slowly add veggies, fruit, full fat yogurt, mashed beans … Add butter or oil if you are concerned about weight. A second opinion from a different doc may give you more confidence .
On a side note our pediatrician did show concern when he was 18 months and had dropped off the charts in weight and had gone from the 80-90% in length to the 28%. Then we were told to remove water from his diet all together. They ran a bunch of tests, and guess what? He is just skinny (like his dad)! Now at two he's at the 4% in weight and 76% in length. Growth charts are just numbers, and every baby is different.