It's not the age gap that's the issue here, and 18 v 21 is fine; it's the level of maturity.
When you say you are 'getting ready to move on' you must have talked things over. On the financial side, as a homeowner I've posted this list of costs to consider when moving out, so this is a starter to discuss with your boyfriend about what you can afford to do:
• Accomdation costs
• Contents insurance
• TV and licence? Stereo?
• Broadband
• Phone
• Food
• Toiletries
• Cleaning and laundry products
• Clothes
• Socialising (including travel, food, drinks) and holidays
• Visits to friends and family (including car insurance, fuel, tax, servicing, MOT if applicable)
• Any additional furniture or appliances like kettle, toaster, microwave
• Bedding, towels, tea cloths
• Crockery, cutlery, cookware, kitchen storage
• Birthdays and Christmas
Plus there's all the discussion over joint finances, who does what jobs (like shopping and cooking depending on your schedules) etc.
If your boyfriend wants to rent to see how it goes, and he's spending his money, then he's got a say it where it goes. What's the issue with where he's talking about, does it affect you for studying, job, proximity to family and friends? Moving in together is a big step, so why not see how it goes if you divide your time between home and his place: it's a very different thing to have responsibilities for helping run a home at 18 if you're not ready.