Next time when you permanently give shares upfront (no vesting period) be very sure what you are getting into and even consider formalizing the nature of the relationship and the level of expectation in exchange for those shares in writing (very least in email)
I think the best you can do is have a constructive but frank discusaion with the co-founder.
Asking him if he would be willing to come to arrangement to part ways but reconsider (at least part) of his shareholding since he will no longer be participating and you will need to replace him.
You could also try to get at the bottom of his lack of motivation (though is motivation is missing its not something that you can give to someone else)-
Rather try to figure out of this is a temporary slump, or a long term "new normal".
Also find out if something has changed in his life circumstances making it harder or less ideal to make time for the startup.
Once you have the facts, then at least you guys have a basis for an honest discussion.
Partnerships are NEVER easy.