Little Lucky sick

This is my first time on this forum so ehm hellow, 🙂

Ok here comes a long storie (sorry)

Me and my boyfriend rented a house for 3months ago, we noticed that the man lived behinde us had a very skinny dog, and with skinny i mean something that could break in 2 at any moment, i could stand on 2nd floor look down to him and count his ribs. I asked the man why his dog was so skinny and he told me that he was taking care of it for a friend (Riight…) and the friend would pick it up in a month or so, a month after + some days the dog was gone, tho some of his stuff was still in the garden. Anyways, a few days later we saw 2 very small kittens (now we know that they where a few weeks old like 2-4weeks) well they where outside pretty much all the time (also in -10C) we kept an eye on them both since we didn’t trust the man with animals, then 5 weeks after we saw the kittens we suddenly got visit from a big white/black old male cat, very sweet, was always looking inside and our own kitten (called Benz, got him from a shelter)one day i was alone home in the evening, and i hear a horrible cry coming from outside, i run to the window and see the old cat, but he does look wired, so I open the door and smells right away what that is wrong with him, he is coverd in poop, he dosn’t wanna lick it off but he still do it (I assume he didn’t like the smell either) I say fuck it, take him inside get cover in shit myself too, go upstairs and clean him, then i dry him, go downstairs, gives him some food and hope him and Benz isn’t gonna kill eachother, they don’t lucky enough, they just starts to pee everywhere so I decide to let the big male out, put food out everyday for him and if he needs shelter I’ll open our gardenhouse and he can sleep there. 2 days ago, Benz go nuts infront of the window, I look outside and see the small cat from the house behinde us, he emptys a full boul (w/e its called) of food under 3min, after that i though ah well he is gonna be full now so i’ll just fill it again for the other cat, but I was wrong, he starts to eat again like he never ated before, when i look at him he looks very skinny, I wanted to touch him and feel how skinny he was since the fure make them look bigger and i was shocked, he was more skinny that he look liked when i took him up it was nothing but bones, his powls are bigger than his legs and his head is the biggest thing on the whole body, me and my boyfriend are animal lovers, we can’t stand that people leave there pets to themself, i put the cat down on the ground goes inside and talks with my boyfriend about what to do, we go on the internet to see what can happend if we put 2 male cats together, what that would be best and so on. while he was checking that i was looking at the kitten, and it made me sick, his butt was big and red, kinda reminded me of a monkey butt, he was shaking, drity and just crying. We decided to take him to a vet ASAP, she told us he is 3months old, got flues and are very underweight, just under the 2kg, she gave us some pills we could give him for his stomech called Finidiar, when we came him we made a room rdy for him (for his own and my other cat sake) i spend alot of time there, cleaning feeding him (4times a days on small potions) i wake up at the night 3-4 time to check on him that he is still alive and so, but then yesterday when i was cleaning his room (pooing everywhere, even on the wall, don’t know how it even got there lol, I don’t mind by the way) i saw small blood spots, so he is bleeing a bit out of his butt, What to do ? Shall we go to the vet again? Is it because he eats real food for once? Or got bad stomech? I really want Lucky as we call him to make it, he is a strong kitten (stronger than the other that might be dead since we didn’t see it for ages) but he have been though alot, tho he is a better place now (nice warm, safe place with food everyday and always someone around him) Hope you guys can help us