M.B.
I think the suggestion to look at Celiac is an excellent one, as both my boys were not diagnosed until much later and it is currently way under-diagnosed by the medical community as alot of doctors were taught it was rare (i.e., 1 out of 9600), whereas recent research shows it is quite common 1 out of every 120. There are millions that have not been diagnosed.
That said, you don't mention whether the baby is getting formula or breastmilk. My kids are now older (almost 11 and 13 1/2), but my understanding when they were that age was that the majority of the baby's diet/nutrition until age 1 should come from the breastmilk or formula. Also, it seems to me that some of the recommended foods are ones that can be quite allergenic and shouldn't be introduced to a child under one - eggs in particular.
My oldest son had very slow weight gain (he was always at the 18-20% until he was 4 years old) for two reasons: 1. He was breastfed and breastfed babies do not necessarily gain at the rate on the charts that were developed by the formula companies. 2. He had undiagnosed celiac disease.
However, I was just checking my younger son who gained tremendously early on because I was so worried he would not gain enough like his brother and his weight gain slowed tremendously once he got mobile. He went from 85% to 25% between 3 months and a year. So, whether this is a problem should also depend upon earlier weight gain. If your son also did not gain well earlier, then definitely there is a concern. But, if the slow weight gain is new, then I wouldn't worry. My younger son kept at that 25% for over a year and then he gradually worked his way up to his current 90% in weight and about 80% in height (he's now 11) - both my kids height and weight has never flattened out like the curves show they should.
I would also look at family history and see whether your husband's or your family had large toddlers or petite ones. The pediatrician said my older son would be petite based on where he was at two years. He's now about 5'10" at 13 1/2. His shoe is a size 13.
In short, I wouldn't be too worried unless the baby is also seeming to have less energy and doesn't seem happy and alert and I personally would avoid adding some of the foods to avoid potential allergies (unless of course there's absolute no history of any allergies at all in either families).