While the two of you making the effort to help his parents is admirable, it may be exactly the wrong thing to be doing. Helping his parents help themselves is a better way to approach this. The plan should be clear and with a distinct timeline that you are going to move out on a certain date and time — and that they will have to start looking into other sources of getting their needs met, such as social security disability, vocational rehabilitation, etc. If you allow yourselves to be an endless funding source with no plan of extracting yourself you may be enabling their lack of motivation to change. In other words, you might be the very thing that is keeping them stuck.
I would highly recommend a family or couples therapist to help you extract yourself from this situation. Supporting his parents at 25, with no plan to stop, will perpetuate the situation rather than resolve it. You can find a therapist near you by clicking the find help tab at the top of the page or you can look at people registered with this organization. I’d rather see you struggle with feeling a bit guilty for taking this path than feel resentful for the rest of your lives for not. The time for change is now.