Can I start my 11 month old on preemie formula for the extra calories?

My son is only 14 pounds at 11 months, his birth weight was 9 pounds. My Ped is always suggesting trying to get extra calories by adding Karo syrup to his bottle or other sugars. He is been on Similac or Enfamil regular formulas and tolerates them well. I guess I never realized until today (Saturday Ped is closed) that they made preemie formulas that provide extra calories, I was thinking of maybe buying a can and mixing 50/50 with his reg formula but can't find much online about these special formulas being given to an older child on formula. I plan on giving him formula past his 1st bd in the next few weeks. He has been pretty easily switched between Enfamil and Similac either one is fine for him so I don't think adding something new would bother him. Has anyone ever done such a thing? I would think it would be a healthier alternative to corn syrup. Obviously I will call his Ped on Monday but just wanted an opinion if I may.