Hello Kat and thanks for your question/post.
First, it's hard enough dividing property among married couples, so dividing property among unwed couples would be extremely difficult and complicated. I'm going to err on the side of assuming that, basically, your boyfriend is a decent person who truly wants to help you in your home purchase, and also understands now may be the best time to do that in California. As such, he wants to protect his downpayment and investment in your home by requiring that his name be placed on the deed. This way, if you should default on payments to him or to the mortgage company, if there is any money left, he would be entitled to the fees. It is only natural that if someone invests a large sum of money in another person's home that the best protection is to be named as a co-owner.
Now, the bad news...
As the other Realtors have mentioned, if there are two owners for the property, the property cannot be refinanced or sold without the consent of BOTH owners. If, for example, you've decided to move out, sell the home and payoff your boyfriend, and he does not agree, you cannot execute any listing or sales contracts to leave the property, and can, in essence, be held captive until the other owner agrees to sell. If you have any foresight that he may be difficult or present problems if you want to sell, then he should not be on title with you.
Also, determining how much "ownership" to give could be a problem. If you purchase a home at say $100,000 and he gives you $10K for a downpayment, would he be happy to accept only a 10 percent stake of ownership in the home? Probably not.
My suggestion is that you talk with a qualified mortage broker to determine how much home you can afford, and the funds you'll need to put as a downpayment and to cover closing costs. As you are first time home buyer, the FHA (Federal Housing Authority) provides loan programs to first-time home buyers with FICO scores over 620 and a down payment of 3.5% of the sales price. If you can qualify for these programs, it would make a home purchase possible without help from any other source.
Whenever possible, for maximum control of your own financial well-being, it's best to keep your money separate from those of boyfriends and family members.
Good luck in your house hunting!
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA