How can I obtain joint custody, legal guardianship, or some sort of parental rights to my roomate’s son?

This is a slightly unusual situation in that you do not want to get married, just obtain some type of legal status as “dad.”  Before you make any final decisions, you need to understand that once you step into the dad shoes, there are long-term consequences.  Courts won’t just deem you “dad” and then let you walk away.  You do have a couple of potential options depending on how much status you want and how much long-term liability you are willing to accept.  The lowest level of status is to have an attorney draw up some type of power of attorney that allows you to make decisions while Mom is out-of-town.  For example, you may want to have the ability to consent to medical treatment, especially if she is on the road frequently.  The middle level is to file a petition with the court to have you appointed a joint conservator.  On your own, your “standing” to file for custody of the child is somewhat limited, but if the Mom does not object, then standing should not be an issue.  (Standing is your right to file a particular cause of action.)  As a joint conservator, you would have all the rights and duties of “Dad”, but not the legal status.  The highest level of status is to petition for adoption of the child.  Not only would you obtain all the rights and duties, but you would officially become dad.  You really need to think carefully about this option.  You can divorce a spouse, but you cannot “divorce” a child if you have second thoughts later.  The power of attorney option is not going to be that hard.  The latter two options, (conservatorship and adoption) may be more challenging depending on the attitudes about your living arrangement in your jurisdiction.  Some judges, quite frankly, may have concerns about granting such extensive rights to some who wants to be dad, but not husband.  If you are really serious, you should visit with an attorney that regularly practices family law where you live.  They would be in the best position to give you a feel of what the judges in your area will approve.