I believe that every entrepreneur eventually has gone through a similar situation, the decision is not easy but yet it is not difficult to answer. In my opinion there is now capable people at very young ages and that must always be one of the reasons why they offer more advantage over some other team members even when they have advanced ages. Looking at your question, you mention that the new developer will be considered as co-founder and will be available full time, your current developer was just part-time.
If you're already taking the place of co-founder I think you know the experience that his have, how it should be communicated to the team, I think that you should always be putting the advantages to the resto of the team members of the important to had in the company a person with that ability, experience and knowledge that currently the new developer has.
In summary the facts are those that will give the result of those actions were taken, if the team is mature enough will understand clearly that this new developer gets a larger percentage in stocks Company because the expertise that him or her have, if for some reason a collaborator show anger or discontent I think the shirt was never enough to start taking the project forward.