Your personality sounds a lot like mine; I don't tend to be assertive at all. I used to think it wasn't my place, I should defer to those who had more experience. However, I have had to learn (especially at work) that sometimes I DO know better than the other person and it is my responsibility to stand up to someone to protect a tiny human. It is hard, and I feel your pain, but it gets easier.
If I were in this situation, I think I would tell MIL, "I respect your opinions, but as momma, I am the Commander in Chief for this LO. I have made the decision that he will only have BM until x age, and I need you to respect my authority on this. No more tastes or licks. Period. Thank you for your love and support. I really do appreciate your standpoint, but it is time for you to appreciate and respect mine." Go into the conversation as calmly as you can...that's the hardest part for me. And hold tight to the fact that you are making this scary step in order to protect your baby. In the grand scheme of things, whether he gets food at 4 months or 7 months is probably not going to shape his entire future, but that's not the point. The point is that you are this child's first and best advocate. It starts out with feeding issues now, but eventually it can turn into bigger things. Standing up for your child's needs does not make you a B. It makes you a good mama. MIL might be surprised or even a little put off for a bit, but deep down as a fellow mama, she should recognize and respect that you are doing what you feel is best for YOUR child.