Frankincense would be very good. A drop or two in a glass or water 3 or 4 times weekly, and obviously monitor the results. It is said to be good for ulcerative colitis, and excellent for boosting the immune system.
Bergamot, rosemary verbenone, and cineole-rich oils such as cajeput are also useful for boosting the immune system.
Hope this helps, and let us know how it works out.