[Edit - thought he was a current supervisor the first time I responded, apologies]
Given he's a past supervisor, you have some leniency on if you use him at all. If I had to guess, I'd say you're asking this because he thinks highly of you and you feel he would provide a strong content recommendation (barring execution). So if you do want to use him, it comes down to managing the process. For one, he needs to be explicitly prepped. This includes themes, experiences, tone, etc. Additionally, it may really help here to provide him with examples (if you have them available) of "great" recommendations to give him an idea of the level he's now playing at.
Look - I know you'd just as soon not have him write anything but if he has the capability to help your story, it will come down to how do you set him up for success. There are numerous ways to do it and you might have to use them all!
I hope that helps,
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