We both want him to heal from this breakup in a healthy way, and want to give him the space to deal with it on his own without having me fall into a rebound-default-girlfriend mode, but are struggling to figure out how to set those boundaries.
What's wrong with keeping it the same once a month deal?
And how about reading the stages of grie
f together? Keep tabs on his processing through them?
And him spreading the burden around so it is not all on your shoulders? Talk to other family, friends for support during his break up grief? It is fine to share some with you, but share with others so the intensity or volume knob isn't ALL broadcast in your direction.
Could also give a time limit -- "We are not getting serious or talking about getting serious or building a future together until a year from now to create the space needed for grief" -- then when the year's up you can check in. Grief may or may not be done in a year. But in the meanwhile you have a measurable
limit -- "Year's not up yet. Let's honor the agreement." When you check in if more time is needed can always make a new agreement.
Just some ideas.