He was a normal weight at birth and appears to be gaining weight okay for all the struggles you're having, thankfully. You're doing a good job with him.
There are various feeders developed for feeding problems and one of them might help your baby. You might try the Haberman but realize you could exhaust yourself trying all these different feeders and to no avail. Don't be discouraged!!
http://www.selfexpressions.com/habermanfeeder.html