It's funny...I've struggled with Body Dysmorphis Disorder (BDD) for many years, though the fused 2nd and 3rd toes of my left foot have never really been an issue for me. However, I can completely understand others' feelings of repugnance or shame regarding their syndactyly. As I mentioned, I have had a long, painful history of trying to reshape a body which, for years, I percieved to be abnormal.
The question is...will plastic surgery to correct the fusion truly increase your son's happiness/life success? Will it increase his self-esteem, or raise his market value as a human being? Maybe. I'm not going to tell you categorically that it won't. But I do know two things for sure.
The first is that recovery from cosmetic surgery is pure and utter HELL. In my own experience, the ensuing pain, nausea, and disturbing numbness (due to nerve damage) far outweighed the potential benefits of looking more attractive. I can't even imagine how painful or debilitating it must be
to experience such discomfort on the body part I use most every day.
Secondly, in terms of social success, self-esteem, and ultimate happiness, your son will benefit much more from you modeling confident behavior that teaches him self-determination, rather than externally-focused behavior that teaches him to be insecure and anxious. I'm not saying your concerns are totally without merit. The fact is, your son probably will be teased as he grows up. If not for fused toes, then for scars where a fusion used to be. Or acne. Or being too tall. Or too bookish. Or too whatever. Take a step back from your situation, and you will see that this really has nothing to do with toes. It has to do with fear, and wanting to protect our children from being hurt by others as we ourselves have been hurt.
It's almost unbearable to think about, but the truth is that at some point, your son will likely experience tremendous emotional pain - as do we all. My prediction is thay the deciding factors in his ability to survive that pain will have nothing to do with his toes, fingers, or any other body part.
P.S. Syndactyly skipped a generation in my family. My beautiful paternal grandmother had it as well. Before I knew anything about inheritence, it was physical evidence that she and I were linked - proof positive that I was carrying on her genes.
Best of luck in your decision!