M.C.
Accidents happen. This was a serious accident, but it's not like your husband purposely set your son down on top of the metal grate. He likely checked the temperature of the playground, and it didn't even occur to him that there were other surfaces that could be hot enough to cause an injury like this. I can guarantee he will be more vigilant after this!
I understand that it is hard to see your child in pain like that. I really do, after watching my (then 3 yo) daughter deal with an idiot nurse who tried to "pop" her broken elbow into place (twice!) by yanking on it. My daughter was arching back and couldn't even make a sound, until she got past the shock of it and started screaming herself voiceless... At which point I removed my daughter from that office and took her to the ER. (And I completely understood that, while her broken bone was preventable, the person who was watching her did not deserve to take the blame. It was an ACCIDENT. My husband still has a hard time letting it go though, and it's been a year.)
Your husband is likely feeling * incredibly* guilty. Seriously, if there is a worse feeling than being helpless while your child is in pain, it's feeling like you are the one caused it. :( you don't have to 100% "support" him, and it's ok to let him know how you feel. Your emotions are not subject to logic, and it will take some time for you to sort through and process. Just let him know that you are trying (and really do try.) this will be especially difficult right now and while your son is recovering. Let him know that in the future, he needs to be sure about injuries, not assume that they are not that severe. (A lot of people, myself included, take a "wait and see" or an "only if you are dying" approach to doctor visits. It is something that I have really struggled to overcome with my dd.) But please don't blame him too much. Also, allow him to tell you how HE feels about all of this. He may very well be covering his feelings of guilt and insecurity in his role as the father (ie, protector of he family... And he failed to protect his son...) with annoyance at you for 'refusing to support him.' Your son will heal, make sure your marriage does too.
I also suggest counseling for both of you. It is HARD to watch your child suffer serious injuries. My cousin almost lost her 2yo son after he was kicked in the head by a horse and suffered a Traumatic Brain Injury. Again, an extremely unfortunate accident that could have been prevented, but wasn't. (These things actually happen quite often. You should hear some of the stories my pediatric nurse aunt has) He was in a coma for 3 days, had to have a metal plate put in place after the pieces of his skull were put back into place-before waking up with tears in his brain. It was over a week before his chances of survival were high enough for his parents to breathe. It has been a year or so since it happened, and he has had to do extensive therapy to regain his speech, and still has behavioral problems and developmental setbacks that may be permanent. He has to wear braces on his legs to walk. She has suffered from Post Traumatic Stress from the entire ordeal. Therapy has saved their marriage, and helped them move past the blame game and into working together to help their son heal, while still showing their older daughter the same care and devotion she deserves.