' He'll get too comfortable' ? What's wrong with that ? Either you want to be a family or you don't. Either you're happy with him being around full-time ,like a proper husband and father, or you're not . Which is it? His putting a ring on your finger won't change that fundamental.
If you don't want that and prefer to live without him,then do that. Frankly, you are using the 'he's afraid to get married' reason as a cover. Even if he gave in and married,the ring won't change him. If he wants to stop living with you, he'll go when he wants, married or not. It's that he won't stick around that worries you, not the lack of a marriage certificate..