I know exactly where you're at

My monster was also born three months early. She's currently 20months actual/ 17 months corrected weighing 16lbs 14oz. Similarly, Talia's bright and active, playful, all there intellectually and developmentally.
I very rarely tell people about her being FTT because it's just not worth it. I hate the label because it completely dismisses her vibrant personality and perfectly normal development. At our 12 month corrected follow up clinic she was perfectly on track and even a bit ahead in some of her development.
I actually dropped my old paed when he gave completely impractical advice for her lack of weight gain. "Get an extra 300mL into her a day"

We got far better advice and support from the nutritionist at the hospital and seeing them was covered by medicare rather than the paed charging $120 a visit
So to the actual useful advice we got. Make every mouthful count. It's not so much about getting them to eat more food, just super-charge the quantities they do eat. This means using full fat milks and yoghurts, creams and cheeses, custards etc. Dairy is good for extra calories. Add butter, oils and cream to his food. Start with small amounts then increase the proportion. Currently Talia gets 50:50 double thick cream to yoghurt/custard as dessert. And I add a good teaspoon of butter to her main meals.
Offer dessert with every meal. Offer cheese for snacks. Some FTT prem mums I know swear by adding protein powder to food to further boost the calories. Choose your brand according to the highest calory/fat count on the label, this is getting harder and harder with the obsessive anti-obesity trends. Talia's breastfed but the formula fed prems I know often have their formula made up with cow's milk instead of water, and/or made up at 1.5 strength.
We had to push our paed to get an answer as to what the actual threat of FTT is. It only really becomes an issue when their head circumference also drops down the charts on growth. At its worst that can restrict brain growth and development, but I think it's got to get pretty bad before it reaches that point, especially given my monster is still a good 1kg below the standard charts even for corrected age and she really is doing brilliantly developmentally.
I wouldn't be too stressed about him stopping using words for now. Keep encouraging him to verbalise, and praise him overenthusiastically when he does talk (think muppet waving happy dances around the lounge room

Children really do love that as enticement to do something). Hopefully he'll pick up verbal play again soon.
I'm more than happy to share whatever bits I've picked up along the way on this

Whatever happens, don't blame yourself. It's not easy being the parent of a prem, especially of the extremely early variety. Your boy is vibrant and healthy and incredibly active (burning through so many more calories). You're doing a great job.
