Not a mum of a preemie but of a tiny - my DD is 11mths old and only weighs 16lbs (same as your DS). Glad to hear you're happy with the weight - he's obviously a solid wee thing. Just to echo the advice not to get too hung up on one thing esp weight - every baby is different - enjoy it and if he's happy, eating, sleeping and developing don't worry.
I had no issues about my wee thing until the PHN intervened and made me worry.
She eats well when she wants is flying on all her developmental milestones and I was happy she was well and happy just little - then the PHN decided she was too little (she was below the 3% line) and referred her via GP to paedetrician. Very anxious time followed - me bulking up all her meals with protein and carbs, dragging out feeding till she ate everything, reintroducing dropped bottles etc. Long and the short of it consultant has seen her twice and says she's a perfect "little" girl. We call her our little doll cos she's just perfect and beautiful but tiny. She's just going into 6-9mth clothes except if they're Tesco's where she's in 3-6mth still
We're going to a party with a load of other babies in her birth club and the weekend and I'm dying to see her beside some of the other babies of the same age but at least I'll be smiling about the differences not worrying.
Enjoy your wee baby.