Enjoy your daughter's wedding! Congrats!
My Jasmine was real small at 7 months old when I got her... 1 pound, 11 ounces... She never had any problems but I used a syringe to feed her meat baby food and plain yogurt for awhile... I think it was more for me than her... I just thought she was too small and not eating much.... I always left dry dog food out for her... Anyway... she's almost 2 years old now and hovers around 3 pounds...
As other YTers were saying I think he should be tested for liver shunt (since he is so small) to rule that out. Wishing all the best for the little guy!!