I think it's a good sign that he's mixing with his friends but that doesn't mean to say that he doesn't value the time he spends with you. You feel you need to keep an eye on him but the more he shows you that he's doing ok the more you can relax and make contact when it suits you both. He knows where you are..Your Mum will always be that shared link but the fact remains that he may be ready to move on with his life and you are grieving for someone you have known all your life.