What normal weight/height gain for 7 months premature baby boy who born at birth 1.67 kg and height at 41 cm. Breastfeeding and now 3.5 months old.?

Needs more calories. You haven't mentioned how premature was your baby,but my guess he was born at 31-32 weeks gestation,at 1.67 kgs,now at age 7 months he is 3.5 kgs,doubled his birth weight,but still needs more,hope his other developmental milestones are fine.Usually premature babies catch up with normal growth after 2-3 months,I am afraid breastfeeding alone isn't enough,he needs more nutrition,see his doctor.