Praise be to Allaah.
If your work is in a permitted field, there is nothing wrong with your working for him, so long as you continue to obey the commands of Allaah. You should also do your duty of da’wah, calling to Allaah in the best way. Perhaps Allaah will guide him through you. The Prophet (peace and blessings of Allaah be upon him) said to ‘Ali (may Allaah be pleased with him) when he sent him to the kuffaar on the day of Khaybar: “Call them to Islam and tell them what they have to do, for by Allaah, if Allaah were to guide one man through you, it would be better for you than red camels.” (Red camels were the most precious form of wealth to the Arabs at that time). (Reported by al-Bukhaari, 2787).