What type of feed is in the bottle? Make sure it's a supplement like KMR Milk replacer (from any pet store); regular milk could be the actual problem.
It is possible that this particular kitten is suffering from some defect or illness if the others are on solids already and he's still having difficulties. I won't lie to you, at that age, treatment tends not to work out well (depending entirely on the problem, mind you).
Well all of them have about started on dry foods and using the kitty litter box but Jr hasn't begun any of that and mom refuses to feed any more. We have to take her and get her to lay down and then place him on a teat. We would take him to a vet but we don't have the money for the bill. We have him on a bottle as well as moms teat.
It's probably to soon to say. As long as the kitten is only nursing, you won't see any bathroom activity (other than the mother licking to stimulate urination).
Runts often develop slow at first and grow into healthy cats. They tend to be on a different early development schedule than their litter mates. Once Jr reaches an age where he can eat solids, he'll likely flourish and grow.
Just be sure that Jr actually gets feeding time at a teat. Make sure he's actually getting milk as well, and not just "dry nursing". I'd go so far as to taking him and the mother aside seperately and ensuring that he's getting all the food he needs.
If you're truly worried, I'd take him to a vet either now, or a few days after he starts on solids.