Great kid with great ECs. Definitely needs to write about his care of his brother in his college apps- that is an “EC” just as needing to work instead of joining clubs. Being an Eagle scout involved a lot as well. Do not worry about his nonacademic time- he has shown success. I think part of the EC bit is to show a student can handle more than just the schoolwork. You emphasized academics- kudos to you- and your son has done much more.
Someone mentioned the effect of his leaving home for college. That is part of life. You sound as though you plan on it- great. His choice of schools to apply to will be influenced by family finances- do not presume there will be merit money at the schools of interest to him. His choices should not be made based on coming home to help with family, from your post it sounds like this is not in the plans. Be sure your son understands that he has no family obligations when it comes to comes to college. He needs to get the best education for him, not one that includes caring for his brother.
There are so many good engineering options. Be sure you do not fall into the trap of thinking only the schools listed by CC’ers are the best. Many public flagships have top programs- and where you look depends on your state. You also should be sure he is not locked into a major- who knows, math or physics may be more his thing and he may decide after starting college. He will find plenty of academic peers at many top tier schools, especially in fields like engineering.
Quality, not quantity. Your son hits the mark with both of his activities. The only thing you might do is ask him if there are any activities he would like to do if he had the time instead of taking care of his brother. Eventually you will need to outsource care taking of your other son so now could be the time to consider how that will be done. Easing your oldest son out of this job may also make it easier for him to transition to college without guilt for not being there.
Sounds like you are doing a great job- balance for both kids.