That is great news that your father has been released! Great news!
Eating after transplant can be a challenge on a number of fronts. For me, I loved food with a lot of spice and heat, but could not stomach them for months after transplant. I found myself eating and craving foods that I had never really enjoyed like peanut butter and chocolate (i know that I am odd here). The trick is to find the food and drink that works best, and then work on rounding out the diet over time. The nutritionist will have a lot of good suggestions. I totally agree with Neil's assessment - your father would not normally be released unless the doctor felt like he could thrive outside of the hospital, so worry is probably premature. The overall trend on weight and energy is the big concern.
Also, maybe too obvious, but 10 pounds of weight loss in a week usually indicates a dehydration type process as you pretty much cannot lose 10 pounds of muscle or fat that quickly, so make sure that he is getting adequate fluids and retaining them. I had a period of time where I went in every 2 or 3 days to receive fluids until my body was more capable of retaining them.
Lastly, and please don't take this as anything other than a sharing of my own experience, make sure that he gets to all of his appointments as that is the key to long-term transplant success. The doctors saved my butt at least 5 times over the past several years since transplant as they recognized problems that I would normally write off as "just being a little sick". I saw the doctor so much (lots of complications), that there were times that I just didn't want to go - fortunately my father-in-law was tough as nails on me and drug me out of bed to get to the appointments regardless of how I felt.
Best of luck going forward. You seem like a great advocate for your father!
MDS RCMD w/grade 2-3 fibrosis. Allo-MUD Feb 26, 2014. Relapsed August 2014. Free and clear of MDS since November 2014 after treatment with Vidaza and Rituxan. Experiencing autoimmune attack on CNS thought to be GVHD, some gut, skin and ocular cGVHD. Neuropathy over 80% of body.