IMHO if your product and company works, you're in it for a long time. That means your co-founder should keep their motivation long-term as well.

If your first sale happens within the next few months, I would keep a 50/50 split just because they will put in a lot of effort in the future. Alternatively, you could start your vesting now and and the other founder could start vesting as soon as he joins full-time. Not sure if this is legally possible.

Another option would be to count the difference in time you spent on it and translate that into equity and ask your co-founder to give you that (like 45% vs 55% equity split). I wouldn't go more than 5%, since I prefer to keep things even. Again, you've got a long road ahead of you, so make sure both of you feel comfortable with the split and with future obligations as well.