Border Collie acting weird, is it his heart?

My 10 month male border collie loves to play outside, however after about 10 min of play he has episodes where he becomes disorientated. His front legs begin to give out causing him to face plant into the ground. He weaves, leans to one side, and becomes gly eyed. I rush him inside and after laying on the cool floor panting for 30 minutes he's fine. The vet did blood work and an urinalysis and both were normal. His best "guess" was hypoglycemia. I've increased his carbs and feed him 20 - 30 minutes before going out. No improvement. Could he be having a cardiac episode? Do they do echocardiograms on dogs? Should I seek a second opinion? It's scary to see and I just want to make sure he's ok. Thanks for any input!