B., you remind me of me just a few years ago. While in my situation, I was married and my husband helped with the discipline, the rest of your request sounded incredibly familiar so I will share with you what I've discovered with time.
Fathers are often manipulated by their daughters. The daughters manage to bat their eyelashes or crawl onto daddy's lap and hug their way to getting what they want. Sons, on the other hand, manipulate their mothers. It isn't as overtly obvious and certainly not so clearly defined, but it does happen and is the cause of many a hard feeling on the part of the fathers. While we (the mothers of sons) are being treated one way by our sons, the fathers and step fathers are sent an opposite set of messages. Just as it is with daughters trying to take first place with the father, sons try to fill that role with the mother. The father’s response is often to feel attacked. Not only do they feel their role in the family is at risk, their masculinity comes into question too. Each time I argued that my sons were just being boys (or whatever argument I had in their defense at the time), my husband's feelings were dismissed and I was reiterating to both my husband and sons that my husband’s role was not one set in stone.
Your sons, at ages 9 and 12, are at the cusp of spreading their wings and becoming men. What this means to you is that over the course of the next few years, they will be continually testing the waters and seeing if they can step into the role of your husband. You will not always see it happening. You may never see it. That is the way of things, but the fact that it is happening won’t be changed. Behind your back, your sons will push limits with your boyfriend, make comments to and about him and may even try their hand at sabotaging his belongings and the relationship between the two of you. Had someone shared this with me, I would have been saying, “Oh, my sons would NEVER do that.” I would have been wrong just as every other mother on earth would be. Its part of their development and as wives and mothers, we need to be able to recognize it so that we can help our sons become men while ensuring our husbands feel as little of the growing pains as possible.
Talk to your boyfriend and find out if any of what I have said rings true for him. If so, I encourage you to listen, listen and listen some more so that you can do everything in your power to be a supportive mate. He’ll be able to help you see some of what has been going and because of having been a son himself and will be able to help you know how to handle the situations with your sons so that both he and your sons feel supported.