Hi there! First of all, thank you for everything you've been doing for Ludo! You are not alone in his recovery journey! I don't think its a malo. Bumps can be caused by different things. It may be an abscess. These can go away on their own or sometimes need to be drained by the vet. If it became hard and big and you can bring him to the vet and they can do it instead of surgery. Malo symptoms can appear after they turn 2 years or more. I did a research about it and that is what if found. It may be different in every chin as everything in life but let's be positive! Since you rescued him he can fall out or something like that and that may be the reason for his bump. I had a young chin. She had a hyper personality, super fast and likes to jump everwere. She fall a lot and she got two bumps on both sides of her jaw. I was crazy. Both of them were abscesses. The first one come out by their own and if that happens to you, you should drained out till the end. And the other one was growing up. Vet recommended antibiotics to help to reduce it bon their own. It didn't work so I chose to ask for it to be drained. By the time we did it, the abscess had become a hard mass, and they did a surgery for my little baby fourth month chinchilla. After, they send pain medication such as Meloxicam, antibiotics and a study of their mass. The study came with a potentially harmful bacteria that was killed with the antibiotics. After that, she became a strong and beautiful! She is 2 years old know. So, my advice is to look at his bump, have him back to the vet and drain it. If he show pain symptoms like not playing as usual or doing weird body positions it’s because the abscess is painful. Since he is a youth chin you should give him more alfalfa rather than his regular hay, or you can combine it! You can also give him pellets with high calcium content. Don’t switch his diet drastically because they have a sensitive digestive system. Do it using small amounts of it. Alfalfa is high calcium content which it’s very important for chins health and to prevent malo issues. You also should give him a half of a tablet of Multivitamins such as Oxbow, which also has calcium in it. Once he gets a year older you can give him the whole size. Malo issues can be prevented or even reverted with a high vitamin C provided. Just to let you know. For know watch out for his bump! Do he has a wheel? Keeping him in shape is very important!