I am sorry to learn of your loss, please accept my condolences.
It is important to keep in mind that your fiance's assets and debts became the responsibility of his "estate" when he passed away. (They are not your personal obligation).
Your fiance's estate is responsible for the payment of his taxes.
His assets (including his partial interest in your home) can be used to pay for the obligations of the estate. However, it is very rare that the IRS will levy on a primary residence to pay for a tax debt - they may place a lien on the property (that will be satisfied when the property is sold or refinanced), but they rarely actually force the sale of the home (especially when the tax payer (your fiance) only has a partial interest in the property.
I would recommend visiting your local court's self help desk for assistance in putting your fiance's estate together (given what you posted here, it is possible that you can utilize a short hand procedure for small estates, but with a real property interest involved, you may have to actually open a small probate procedure - talking to the court's self help center will help you with this.
(If you cannot get assistance at the court, contact your local (county) bar association and see if they have legal aid or legal clinics that can assist you - most county bar associations have attorneys that volunteer a couple of hours to help people with some general advice and can help by looking over your documents to see what exactly needs to be done to help you complete these matters - it is outside the scope of this forum, and while I think you probably could handle this on your own, I think it would be worthwhile speaking to someone in person and going over his papers to make sure you are on the right track (you are dealing with several areas of law - tax law/probate/pension/real property and putting all of them together can be a little complicated).