Ego is energy, so you don’t want to reduce their energy. You want to add humbleness in ways where this person needs the help of others for their own individual success. Two things to do. First, give them a project or an initiative to drive where they are not the expert and have to rely on others. Second, have a discussion that being an individual contributor will limit their career success, and learning to work with and influence others will be key for their long-term success.