As other's have said, you can't save him. You can encourage and support his efforts, but only he can decide to make the necessary changes. Here are some ideas you can try:
1. Encourage him to get a physical exam and a full blood panel. He needs to know what health issues he may already have and, sometimes, getting a stern warning from an MD is a wake-up call.
2. STOP buying or eating junk food, fast food, snack foods, sweets and treats when you are together. A person who is over 300 pounds probably is enjoying a lot of low-nutrient, high calories foods. Even if you can eat these foods, you need to stop enabling him. So no more take out, pizza, drive-thru, etc.
3. Get off the couch, turn off the Netflix, and get outside. Even if you are just walking in a park or zoo, it is healthier to be moving.
4. Invite him to join you in doing exercise. You can follow a DVD exercise plan at home. No gym or equipment required.
5. See if he is willing to read some books with you. Reading some diet/nutrition books like The Plant Paradox, Bulletproof diet, The Whole 30, Stop Eating Your Heart Out. The Keto Rest Diet, Extreme Transformation: Life Long Weight Loss in 21-Days, or many others. Once people begin to understand the biochemistry and psychology around over-eating and health, they can make wise choices about making lifestyle changes.