I probably wouldn't buy him. Depending on what caused it, either a deformity or lordosis, it will eventually catch up to him and start causing problems. The risk of spinal arthritis will be much higher because his back won't function properly like a normal horse and lordosis is a degenerative disorder, so it will only get worse as time goes on. He's 5 now, that's very young, and that's likely why he hasn't had any issues yet. BUT, how will his back look/function when he's 10, 15? Will he even be rideable then? Especially under a heavy work load like he would have in eventing. IDK, to me, it doesn't seem prudent to invest in a horse with such a serious defect, especially considering you don't know how it will effect him in the future. He could be sound for another 20 years or he could be sound for only 2. It just doesn't seem worth it to me.