Alfalfa/lucerne is generally a very good feed. It is very low in sugar, so a good choice for lami-prone horses. High in protein, but it is a much easier to digest protein than what they get from grain or such, so a good choice for oldies who's guts aren't so good at processing protein any more. The fibre is also easier to digest than regular hay or chaff, so also good for oldies or horses with teeth probs.
Lucerne is also high in potassium, so it's important to ensure their diet is well balanced, and it isn't fed as the sole roughage source - lucerne chaff mixed half & half with grass hay or oaten chaff is generally a good ratio for many horses. If you want to put weight on him, ensure he's getting at least 3% of his bodyweight daily in roughage. It is also a good fattener, so it is not necessarily the best for 'easy keepers' & overweight horses.
If the 'senior feed' is grain based or otherwise high in starch or sugar, particularly if the horse is not fed little & often - only fed 2-3 larger feeds per day say - this could be the reason he's a 'hard keeper' as horse's systems are not evolved to cope well with starch & large infrequent feeds. Also teeth could be an issue and he should be attended by a *good* equine dentist at least yearly, but may need more frequent as he gets older.
Balanced nutrition is also important and it may be that he's getting enough calories, but imbalance or deficiency in certain nutrients is keeping him from doing well. I use a service called FeedXL.com to help me balance diets and to have an equine nutritionist 'on call'. I find it invaluable and very economical.