I suspect his low gain, and constipation, hints he really gets too little. Because its rare kittens having mommas milk gets constipated.He's currently doing good now, however just gains about 5-8grams per day.
I noticed that mama cat isn't stimulating them long enough to poop, and so I tried to stimulate him. I think he's constipated. I saw this little poop that's hardened already and he was able to expel just a little of it - dont know if there's more hard poop in him or what. How do I help him poop??
i tried the warm water with cotton pad technique. I cant get all of it out because he keeps squirming
I suggest get him alone time on his favorite nipple, so he is undisturbed suckling. If you suspect he hasnt enough energhy to suckle properly, you can put a droplet of glucose sugar / white caro syrup on his gum, indide the lip... It gives quck energhy.
As you cant handfeed, its difficult to give him extra water, nor extra droplets of olive oil; standard methods against constipation.
You could help him going by submering half of him in warmish water... And rubbing etc. Still water or even running water.
Of course make sure he doenst get chilled out.
Another tip, is to take a glycerin cone, same as for humans with constipation, cut out a thin slice, and put in. This will loose and make softer the poo in anus.