This is where a proper agreement is needed at the start. I would agree that as he wants to change things he should offer to buy you out and if you do not want to sell then he should sell to you at a fair price. How often are these 2 week slots anyway? If every other month then probably not too bad as you will have the bulk of the access - though I do not think it reasonable of him to try to hog it in that period - presumably he will sort out and pay for all the ferrying and make sure it is back at the right time, if he wants it every other 2 weeks then he really is being unreasonable. Whatever you end up doing I would recommend you get a proper agreement signed up for your group, this partner or the next one.