Hi Jasmine, welcome to TDK! So sorry for the lost of the sibling kitten and very happy that you have a survivor to the sad story of losing mommy too.
It’s totally possible, in my experience to have a kitten who is doing all the milestones early while others are behind. Remember, that the guidelines for hand rearing are general ones and are only meant to give you a general idea when you might expect them to do certain things–so they are not set in stone.
I would make the wet food/formula gruel and put it in his bottle, and open up the nipple which will help him learn the taste of the wet food and give him some added nutrition. It should also help with his constipation, as most times kits that are constipated benefit when they start getting wet food.
Don’t fret about the litter box. At this age, he’s still pretty small yet to understand. But the wet food may help with this also, it may give him a greater sense of urgency to go and to go on his own. Make sure that you have a box that he can get in and out of easily and to use non scented litter, and preferrably non clumping clay for starters. Put one of his poops in there and when you put him in let him smell the spot where his poop is and take his paw and scratch in the litter. He will finally figure it out.
I don’t know if I would expose him to the older cat just yet, he’s still pretty small. I would make sure when I did, they would both be supervised. At his age, it’s not going to benefit him having an older sibling that’s 3 months old, but once he is able to move around on his own and is interested in actually playing, using the litterbox, eating on his own, is plenty early to let them learn how to be siblings. They will have a lifetime of living together, don’t rush the few weeks that he will need to develop fully.
Bottle Babies
http://messybeast.com/handrear.htm
How to Introduce a Second Cat