Genetics will play a part in how big or little a chinchilla is. Being he is only 6 months old, just because he may be small now, does not mean he won't become much bigger later. Some mutations or certain lines, I guess you could say, I find are or can be slow growers too. I've had some not even start to really round out till a year to a year and a half in age. What was 600 grams at 9 months old, can and at least for me - has ended up to be as much as 900 - 1170 grams in size by age two... not with every chin, mind you, but certain ones, yes. Just depends on their genetics.
Some chins just don't stop growing even after 2 years of age. If you aren't feeling bony bones sticking out or seeing hunchback(which from the photos he looks fine for his age, but they are not the best photos), and he is given a good diet, he should be fine.
On the other end of it, although your baby does not appear to be this, sometimes you can get a dwarf chinchilla(which is a recessive mutation and all it takes is for each parent to carry one gene for to produce a dwarf.).
Sometimes people don't know it is a dwarf till full grown, because to an average person they appear like every other baby their age...till they get older and stop growing like every other baby their age. The dwarves I have seen are usually between 300 - 450 grams when full grown, and look like a 4 - 6 month old kit even as adults.
I however don't feel your baby is a dwarf, provided by the photos as the face and nose do not look like one. I am just using an example however as to one of the many reasons a baby may not grow like other babies around them that are the same age.