Legal Question in Real Estate Law in California

No. They can only sell the fractional interests they hold.

Although it may not affect your question, you should also be aware that whatever you did that gave your son an interest in the property, most likely severed the joint tenancy and made all three of you tenants-in-common. That is a VERY important distinction in the event of anyone's death, or if you divorce. You should have this all reviewed by a real estate attorney.

You should also be aware that, while they may only be able to sell their share, any co owner could force, through a partition action, the sale of the entire residence and the division of the proceeds between the co owners. This is expensive and time consuming, but can result in the property being sold without your consent. So, in effect, yes they can force a sale, but have to take legal action to do so without your participation.