dear B., first of all, I can hear the heartbreak you are experiencing watching your son go through this... as a loving parent, it is difficult to watch a child in physical or emotional pain. First, I would take some private time for yourself to come to peace with this situation. Many times parents feel responsible in some way, no matter that they had nothing to do with it. If you are saying to yourself, "if only I had..." I hope you can find the love and support to heal that.
If you are feeling bad, guilty, sorry, it goes right to your son, and he will continue to feel sad, guilty, sorry and significantly delay his own healing, physically and emotionally. So first, take the time for yourself, to cry, be angry, release in a safe way.
I feel your son would benefit by having some big cries himself. How many times have I heard parents admonishing a child not to cry? Crying can be a very useful outlet, and goodness knows, your son has every right to cry. So as his loving mother, speak to him, encourage him, to cry, it will help him release emotions, instead of them being bottled up and later surfacing as anger or some illness like cancer.
Once he has expressed his emotions, you can show him all the amazing people, with disabilities who have overcome society's so-called norms, and gone on to play sports, be top of their class, be successful in whatever their field of interest. There are so many examples of people who have been injured and overcome it, to go on and play and be even better than before. One example that comes to mind is a young girl in Hawaii, I believe, who, while surfing, lost her arm to a shark. She still surfs, competitively, with one arm. You can easily search online for sports figures who have had injuries and gone on, after healing, to play better than ever.
Now, as to whether your son "should" play any sport now like soccer, contemplate this deeply. It is an opportunity for a good life lesson here for your son. Yes, Life holds situations where something unexpected may happen and how one reacts to it, and adapts to it, is what is important. Teach your son that Life can throw curve balls. Let him know that months of healing time without sports, is better than a lifetime with pain, with further complications, and possibly taking longer to heal, if not causing an infection that could require amputation of the remainder of his finger. And who wants to go through more pain after what he just went through? The suggestion a responder made of wearing a metal splint over the finger will not do. In soccer, there is physical contact with one's opponents, people run into each other all the time (jammed finger sounds incredibly painful to me on a raw non-healed wound) and soccer players do use their hands to play. In the heat of the moment, he will not be thinking about his finger and could reinjure it. Have you ever experienced, personally or someone you know, fracturing an ankle or arm and it is in a sling or cast and everybody and everything is bumping into it? Really think deeply about short term vs long term gains and plain common sense.
You can see this temporary sitting out on sports to develop and focus on another area that your son could have missed by being so singularly focused on sports. What if it turns out, he has another latent gift, painting, art, photography, creative writing, that he would never have uncovered if he only immersed himself in sports. This is teaching him he has many aspects. If you listen to anyone in sports, it is a sort term career, and the wise ones develop and prepare themselves in other ways so they don't fall apart (financially and emotionally) when their career ends.
I will hold you and your son and any other family members in my prayers, that you each heal from this, and receive the Peace, the deeper healing, the opportunities, gifts being offered. Blessings to you all...