Is there any way my husband can find out that I had sponsored my mother?

The story doesn't say whether you are already living with your fiance, or whether you will move in with him after the marriage or earlier. IF he lives with you during the time you are filing for your mom, be aware that the I-130 receipt notice and I-130 approval notice will come to your home. You must make an affidavit of support for your mom, and show your tax return. If you are married with him at the moment of providing the affidavit of support, his financial information and documents will need to be included in the paperwork, unless your income is sufficient. (I have concern because you stated you are a student). IF you do not live with him during the process, he might not find out. However, I am a little puzzled...What's so wrong with helping your mother? Your fiance should admire you for this.

If you plan to file for your mother next year, you will be required to submit an affidavit of support. If your income in sufficient you would not need to include your spouse. You advise that your marriage would take place in 2-3 years. The only time the Affidavit will be needed is upon filing the Application for Adjustment of Status, i.e. Upon turning 21. Please note Affidavits are generally valid for 10 years, but the matters are generally confidential. I would advise however to start that marriage fully disclosing everything and if your fiance can't understand you wanting to petition for your mother, then maybe you need to consider marrying him. Just saying!