My thoughts are with you during this very frightening time.
Your father probably needs to go on a higher calorie formula to prevent losing any more weight. I use TwoCal by Ross, and I also use a feeding pump at night, which is covered by my insurance, luckily. That really turned my life around, as I was unable to tolerate Ensure and had to consume so much Boost to make up the calories that it made me sick and I could not maintain, much less, gain any weight. It's expensive, but worth it, as it contains 475 calories per 8 oz. I have also heard of Carnation VHC, which has even more calories, but I have no experience with that.
I have recently started taking SupliMed for one "meal" a day. It is a fine powder that you can mix with water or whole milk. I mix it with Silk Soy Milk for a total of 608 calories per serving. It's pretty expensive and you can only buy it directly from the company online, but to me it is worth it, as I got down to 99 pounds at my worst and I'm 5 feet 5 inches. I'm desperate not to lose any more weight and would like to gain a few more pounds. (I'm about 105 currently) Since I just got started on it, I can't tell you about the long term effects on weight gain, but I am excited about the product because it doesn't clog my tube (I have a small button-type tube) and it has no bad digestive effects. It's not any kind of pyramid scheme/snake oil type thing, so far as I can tell.
Here is the website: http://www.suplimednutrients.com/about.php
I also take the supplement Juven, which was recommended to me by my doctor. I buy that off of ebay rather than from the manufacturer, to save a little money. It's supposed to prevent losing muscle and again, I have been pleased by it.
It's very frustrating to be on a feeding tube when there is so little information out there, especially when you live in a smaller town, as I do, since the doctors really don't know what to do with me. I have been on one for two years from bad radiation side effects (thyroid cancer which spread to my lymph nodes), and there have been times I want to tear my hair out in despair. I would also recommend going to the website www.oley.org, which is for people with digestive/swallowing problems. It helped me find a GI, who replaces my feeding tube periodically and got me a much better tube for me (button tube with extension set).
Please feel free to PM me if necessary.