I am so glad you told him what was in your heart to say. If you stayed out of guilt or other type manipulation, you would have been resentful in about 30 minutes.
You are so doing the right thing. You might think about telling him that you will support him by celebrating his 30-day, 6-month, and 1-year sober anniversary. You might tell him that you will come to the rehab on visiting days. You might tell him that you wish to allow him the dignity to figure out maintaining his sobriety by himself and then do it on his own - that way he can be proud of himself, and so can you be.
But, really, you hit the nail on the head when you said he was not serious. Business as usual.