I'd suggest to stop feeding the dry food. As Simba says, it's difficult to digest and may well be what is causing the vomiting, as well as leaving Bailey constantly dehydrated.
As you say he needs to put on weight, I'd start feeding the best wet food I could find (except feeding raw, which would be even better). Look for wet foods with no silly ingredients like wheat, rice, grain etc. You only want it to contain meat and vitamins, and for it to be 'Complete', not 'complimentary'. Some good brands are:
HiLife pouches, Bozita, Applaws kitten, Porta21, and I'm sure others can add more brands to this.
The ingredients list should look something like this if it's a good food: Chicken (min.60%), Sunflower Oil, Vitamin & Mineral Supplements, Tapioca, Fish Oil, Seaweed Extract, Chicory Extract, Green Tea Extract
HiLife also do a senior version, but I've not seen that in shops. You should be able to buy it from their website though.
Good luck
