Erin,
perhaps your efforts to support you Dads weighloss by testing different diets/recipes. It is quite amazing how even basic changes can add up over a period of time, if it is incidental though, i would think that it would slow down. And i guess to be safe it is worth running it past your Dr. You mention that you are worried about Diabetes, in general type 2 diabetes ( the more common type to be dx in adults) is usually associated with obesity and weight gain, losing weight would usually improve things. Type 1 used to be called juvenile onset, is associated with weightloss, but also usually excessive thirst, excessive urination amongst other things.

some medications such as some antidepressants, and others such as neurontin, pregabalin have been know to increase appetite, so if you had been on some of these and stopped you could lose as reasonable amount of weight.
I have the opposite situation, by the end of 2006 i had gotten up to 385 pounds! After a relapse i was having more trouble transfering from chair to chair ( i am wheelchair dependant amputee and the relapse severly weakened by remaining leg) i was aware that my independance was at risk so i decided i needed to get to and lose the weight, i have lost 125 so far which has helped no doubt, but i still have another 100 or so to go!