[Lorraine]

A 5 months old Yorkie at 2 1/2 lbs is still pretty small. I do hope you are leaving food and water down for him all the time especially as teething is going to be going on creating more stress. All my dogs and puppies have food and fresh water available to them all the time.
You aren't going to be going on long walks with such a tiny dog unless you are carrying him a good part of the way, he is too small for that amount of exercise.
As for patella luxations, you must be very careful and definitely seek more opinions. Many Vets do NOT know anything about them and love to say any small dog coming into the clinic has a knee problem when it isn't. You don't want surgery that isn't required. Observe him when he walks. If he isn't hiking up a hind leg every so many steps, he is likely fine.
In my opinion, I wouldn't be concerned that he is 2 1/2 lbs at 5 months. 2 1/2 lbs is too tiny for a full size Yorkie and you run the risk of a bunch of problems when they are that small. Yorkies are not meant to be that tiny and a lot of times are small due to health problems, genetic or congenital, including liver shunt, other liver/kidney problems, glandular problems, enzyme problems. If he is healthy and hasn't had hypoglycemia, be thankful.
I have two female pups that are undersized, one is 2 1/2 lbs one is 3 1/4 lbs at 8 months. I gave them free to friends of mine that have gotten a Yorkie from me in the last couple of years, live nearby so I can help them anytime they need it or take them back if necessary. They are aware of the needs and potential Vet bills that come with these little ones, although I have had Liver Shunt and blood testing done at my own expense and will be having them spayed at my own expense hopefully next month if my Vet feels they are ready. I do not breed for undersize but it happens in the best breeding program anyway.
Hope that helps a bit
Lorraine
Loriben Yorkies