Is the developer outsourced? Not always giving equity boosts the moral or productivity when costs goes down. He might be interested in equity as something he earned during hard work without reducing costs of development. I think the most important question is - what type of person the main dev. is.
Maybe giving funds for dev, he might start looking something on the side waiting for right time to move to something else, with or without equity.
My advise would be to make good sense of what kind of person he is and what motivates him before making any proposals - even mentioning equity until you are 100% sure this should be put on the plate.