11lbs in 4 weeks is excellent but he needs to be realistic about what will happen to his weight loss, going forward its more likely to be 1-2 lb a week on average and he will probably have weeks when he doesn't lose (not sure what happens on Xenecal).
Don't think because he's outwardly confident that he isn't very ashamed of his weight and it inevitably affects how you feel about yourself. It's SOOO visible, can you imagine developing a big boil on the side of your face and learning to live with it? Constantly being convinced that people are talking about it. It can take a lot of confidence just to get up and go out in public in the morning and there isn't always a lot left over to sustain you through a diet as well.
I think self confidence is slightly the wrong way of thinking about it- self-esteem is far more apt. It's not about whether he thinks he is competant to do things (presentations etc), but whether he has value as a person. We live in a society that values beauty and thinness above anything else at the moment. The fact that he's a great Dad, intelligent, nice person etc is really a rather poor second best in many people eyes.
I would say that most people who know me would consider me to be outgoing, friendly and intelligent but they have no idea excruciatingly nightmarish it is to be told to get up and dance at a social function in front of people I don't know, how I live in terror of losing my luggage if I'm on holiday because I just can't go into any shop and replace enough to keep me going for a couple of days because I'm too big, how I worry about whether people avoid sitting next to me because I'm fat - you can't have these kinds of thoughts without it affecting how you feel about yourself.
Someone once asked me if the dress I had on was Moschino - nearly pmsl at the thought that Moschino would be seen dead making a size 26!
SOrry going off the point there a bit - the upshot as WWB has already said - if your DP has decided he wants to be healthy and lose weight, if he is determined to keep going through possible set-backs, then yes - he can lose it for life. He will probably always have issues with food but can learn to live with it in much the same way as an alcoholic can, but he nees to know that his weight is only one part of who he really is not the whole of it.