I am sorry to hear you are worried about Donovan. My name is Rebecca. I am a veterinarian and will do my best to help.
Are the accidents in the house urine, feces, or both?
I see he is drinking more water. If the accidents are urine in the house, the most likely possibilities are urinary tract infection, kidney disease, or diabetes. If he is losing weight, then I would check his kidney function and check if he is diabetic. Urinary infections do not usually cause weight loss.
You could start by taking a urine sample to your vet for a urinalysis. If he is diabetic, there will be sugar in his urine. Then he should have blood tests done if there is sugar in his urine, and also to check his kidney function.
Cushing's disease can cause dogs to drink more and have accidents inside, but usually these dogs do not lose weight. It is still a possibility.
Other less common things to check for are Addison's disease, hyperthyroidism (rare in dogs), diabetes insipidus (different from diabetes mellitus, which is caused by low insulin. Diabetes insipidus is rare, and is from low levels of a hormone called ADH).
I would start with a urinalysis, and go from there probably to blood tests. Please let me know how he is doing and what other questions I can answer.