No, Arabs don't have a longer lifespan than other horses - 30-35yo is about average. They are a naturally lighter, finer breed than many types though, so may look underweight compared to some... especially when people are so desensitised to seeing fat horses these days - not a personal comment of course, but worth considering. A horse shouldn't get skinny purely because it's older - unless it's like 38yo & doesn't have any teeth left!
As others have asked, we need more info before we can give you specific advice. Along with other questions, how long did you 'try' all the different options for? What exactly were the 'grains', 'mash' and how much of these did he get daily, over how many meals? Has he been scoped or treated for ulcers? Has he been tested for IR or such? How about nutrition - has he been tested for major deficiencies or imbalances?
Horses' GI tracts are built for tiny amounts of low grade forage going through their system near constantly. They don't cope well with large &/or infrequent meals and don't cope well with starchy or 'rich' feeds very well.
So, assuming teeth, worms, ulcers or other health probs aren't an issue, first & foremost, good, free choice grass hay or grass is vital. If the horse's teeth are too worn to deal with stemmy grass well, you will need to consider a 'prechewed' alternative, such as chaff, chop or such.
While grain is not necessarily bad for horses, it is very high in starch and unprocessed grains are very hard for the horse to digest. Porblems associated with grain/high starch diets are colic, ulcers, hind gut acidosis, laminitis and weightloss. So I would personally choose a healthier alternative to grain wherever possible. If you are going to feed grain, it's important to feed it processed/cooked and in small meals mixed with a fair bit of roughage, over at least a few feeds daily.
Some alternatives to grain for weight gain are beet pulp, lucerne/alfalfa, rice bran, soy hulls and soy or copra meal.