Thanks for using this forum.
The symptoms of depression, nausea and IBS are quite troublesome. The nausea seems to be due to IBS and should improve on regulating your husband's fibre diet and that may have been explained by a dietitian.
The antidepressant Citalopram is a good drug but if he is taking it for a long time with no resolution of symptoms, you should ask your physician to change him to another drug.
Take care.