He's not ready for real food yet. This is totally normal. One of mine wouldn't eat any solid food at all until 14 months. Then he would suddenly eat anything.
Solid food at this age has no nutritional value anyway, so don't worry about it. Make sure he's getting the breastmilk/formula he needs for his nutrition. Give yourself and your son a break from trying solid food for a month, and try again after he turns 1.
If he's 13 or 14 months old and still won't eat anything, then I agree with the others - get a referral to a feeding therapist. But your child is not even 12 months old yet, and every single recommendation is that all nutrition come from breastmilk/formula until at least 12 months of age, and you are not there yet.
ETA: Weight gain between months 6-12 is on average 1.75-2.75 ounces per week (per Kellymom.com). So, over 20 weeks (5 months), 35 ounces (2.1 pounds) is considered to be within the normal range - on the low end, but normal. So I don't think that there is a reason to panic about 2 lbs of weight gain - especially if it's been slow and steady throughout the 5 months. Now, if he gained 2 lbs between August and October, and no gain at all in Nov and Dec, that is very different and a cause for concern. Otherwise, your ped is happy and as long as you continue to give him solid nutrition in the form of breastmilk/formula, I do not think there is a reason to panic. https://kellymom.com/bf/normal/weight-gain/