Doubt if he was adopted by his stepfather. Why would his father sign away his parental rights to his son (which he would have to do for him to be legally adopted by another man) but carry on paying child support for him?
Doubt if you can claim against the drivers insurance company. If he was not deemed to be at fault, or had charges pressed for speeding or reckless driving, why should they have to pay up or the driver penalized?
With all due respect, I think that your wanting to enter your ex's home for your son to take some of his things at time of the funeral, may be looked on by his family as being a tad insensitive. The grief and shock that they must be feeling at that time must be staggering. While you may have ended your relationship with your ex, had no relationship with his family, had to travel a long distance for the funeral etc it probably made perfect sense for you to do it then. But try to look at it from their point of view. The just lost a brother/nephew/cousin. Give them a bit of time and space before bringing up who gets what, even if your son is probably entitled to the lot legally.
Regarding your ex's estate, do you intend to apply to the Probate Office for Letters of Administration to deal with his estate on behalf of your son, his legal heir? Or do you plan on letting his brother do so as your brothers next of kin, who is of legal age?
Perhaps you should give the Probate Office a ring, and find out from them where you stand & who is most likely to be awarded Letters Of Administration. If it is likely to be the brother, then no, he does not have to give you a key. If it you, then you are the legal owner of the home as the probate process is being dealt with, and you are entitled to be given all keys to it. You will also have the responsibility of of dealing with the house, its contents, bills associated with it, the banks, all debts owed by your ex etc etc. It may be a lot of work, but at least you know that you did right by your son if you have valid reasons to suspect your ex's family of doing something shady with his personal possesions. However, they may have a sentimental attachment to the same things that your son has. It could all be totally harmless, and a little bit of time could ease any hard feelings felt. Best of luck to you. It is a tricky situation, no doubt about it.