Fattening a thin TB. Honest opinions on my horses weight..(pics)?

I have had my tb for 14 years and he is now 20. I always had a problem with keeping weight on, but you have to remember that tb's are not meant to be fat I think he looks fine nice coat and you are doing a great job with his feed don't let every ones judgements ruin how you think about him. When my boy turned 17 his back really sunk and he lost weight that i can't put back on, i know i feed him well and do everything for his heath, it just happens. If you are really worried have bloods done.