First of all, he is too old for spanking. I am NOT someone who doesn't believe in spanking at all.. we spanked our kids.. but my son is now 12, and I think he is just about too big to be spanked at all. Our daughter is 9, and she is not too old, but it is truly a rarity that she is disciplined that way. (Almost got one last weekend for spraying the dog with the water hose--totally disrespecting the dog).
It's been a long time since our son has gotten one.
I don't know how much your son is required to do or how much free time he gets, or if he has siblings... but what he has to do (that you've listed) sounds like quite a lot. He has to go deal with the horses in the mornings before school and at night I presume? Also his schoolwork (which in 8th grade can be a lot sometimes). I'm assuming he has to train WITH the new horse.. which while fun, is also an obligation of his time. He works to earn money for his "extras".. and apparently was playing football too. He cleans up the kitchen after every meal? (He does it all alone after EVERY MEAL? Really?) What does his part in "helping" care for the puppy entail?
Honestly, it sounds like a LOT to me. That is a lot for anyone, especially a boy who's hormones and growth are spewing and he is trying to juggle all these daily responsibilities.
Maybe you could have him help with the kitchen/dishes on M, W, F or on F,S&Sundays...
Honestly, it sounds like every hour of his waking day he has some responsibility to manage. No wonder he is rebelling and getting an attitude. I feel that way sometimes about it when it is day in day out with no breaks.
He might have 'plenty of time' for these things... but he needs some "down time" also. To mentally relax. He is just turning into a teenager and needs to figure things out. It is a hard age, even without pressure to remember a lot of responsibility beyond caring for himself. A lot of parents would be thrilled to have their 13 year old just be responsible for THEMSELVES! Let alone the household pet, the yard animals, the laundry, the dinner AND themselves, every day.
He's still a kid. My 12 year old son LIVES for his unscheduled time. If he doesn't get a 4 hour block of time at LEAST once a week (usually Saturdays and Sundays) an a minimum of the last hour before bed EVERY DAY to just chill/veg out... he gets really overwhelmed. They need those times to process the whole week of life (expectations, work, friends/relationships, school, themselves, etc).
Maybe you should sit down and talk with him. WITH him. Let him know that it seems like he is a little overwhelmed with all the responsibility and ASK HIM what he thinks could make things more managdable. What sort of compromises or alterations could make things better? (Don't throw everything out the window and start from scratch, but maybe have him plan on 2 hours on Sat morning doing laundry, puppy care, and cleaning the kitchen/vacumming, etc; and then the rest of the week, he doesn't have to think about laundry duty - other than to have his own laundry in the laundry room and picked up not in the floor).
And the "sneaking" food, that's probably pretty normal. My son is hungry almost constantly... they are GROWING. The thing is to not get mad, but try to direct them to good food choices, rather than junk. So don't keep junk in the house if you don't want to catch him noshing on it. Or only buy chips or whatever once a week, and when it's gone it's gone until next week.
If he is hitting the horses, he must REALLY be overwhelmed. That doesn't sound like he is doing it to spite you guys, just overwhelmed and frustrated. And when he is (presumably) alone with the horses, he is letting it out. So you need to work with him to find a better way for him to lesson that frustration level, and to deal with the frustrations that he does have.
Good luck.