Focus - It really depends on what you want to do. Do you want to talk with him? Find out how he's doing?
Text him back. If you're end goal is the R the M, then it won't hurt anything to contact him back and keep it short, friendly, with no R talk. That's a good way to re-establish pleasant dialogue.
If you want him to have to try harder, respond with the text that you proposed. It lets him know you got his text, and you answered him. The rest is up to him.
The biggest thing in DB is to NOT pursue. Either of those options will be fine. If you're happy with the R ending, then just don't respond at all.
