My dog got bitten by a black widow,need help!?

My dog ,a mastiff -boxer mix got bitten by a black widow about 10 days ago.The next day his leg was really swollen,2 days later he had fever and the last 3 days he developed some kind of bubbles under his skin,it feels like balloons filled with water.I took him to 4 different vets,nobody was able to help.( I am in a rural area of Panama)The dog weighs about 100lbs and is 3 years old.He is in a very weak state now,can anyone help?