I'm going to disagree with you a bit here. While goalkeeping is a little more precise than playing in the field, there are no absolutes in soccer. Like the overcoaching article mentions, coaches need to avoid using terms like "always" and "never". Soccer is the most situational sport with multiple variables you have to assess in a split second. Falling back to the 6 may have been the appropriate decision in your son's circumstance, but never sliding feet first would be taking a very valuable tool out of a keeper's tool box. It's effective, albeit with risk. Personally, I haven't seen it result in as many red cards as you have experienced. These risk/return equations are all things that a soccer player needs to understand to develop their soccer IQ. One of my pet peeve's is seeing a goalie rush out to stop a 1v1 and suddenly stop and backpedal in retreat. That typically doesn't end well as you've released much of the pressure on the attacker and are likely off-balance as you backpedal. Even if you've made the wrong decision to charge the attacker, you're only going to compound the problem by retreating.