How do you effectively set boundaries without offending inlaws?

It's gotten to the point where my in laws need help to do the everyday things. My husband and I are just starting our life together. He gets bombarded with calls from his parents; everything from weekly dinner invitations to repairs that need to be done. He was brought up to believe he has to take care of them in their old age. Well, were there and he will not set the boundaries with them. How can I tell them what we can and cannot do for them? We simply do not have the time to do everything they ask from us. He is fearful any kind of refusal will be taken as an unwillingness to help them and he will be a "bad" son. I need to be able to communicate this to them w/o straining my or my husband's relationship with them because its straining our marriage.