hi G., i am a teacher, both regular and special ed, all grades. i obviously dont know anything about your particular school district, but i can tell you with a fair amount of certainty that they do have your sons best interests at heart. if your son isnt ready for first grade, it is no big deal, it doesnt even necessarily mean that he has any kind of learning difficulties at all. he is very young and kids that small sometimes need a bit more time to get started. and if he repeats kindergarten, he will never remember it later on, it wont make any difference except to give him a chance to catch up and feel self confident and enjoy school. it is much better than always struggling and being behind, which usually leads to a lot of frustration and anger and disappointment and problems with self confidence and behavior, which can change his entire school experience forever. dont do that to him, get him the help he needs now.
if he is now slated for summer school, that is great, make it as positive an experience as you possibly can, tell him over and over how lucky he is, how much they love having him, how well he is doing and so on. do not make it seem like work, keep it fun and light and positive. and get the school to give you everything they have to offer. you pay your taxes, right? use those resources! of course you will work with him at home, learning is a part of everything we do, but it sounds like you have your hands full at home too with little twins! the school probably has more to offer than you realize. develop a good relationship with his teachers (always) and have his teachers give you a lot of specific feedback on how he is doing and what you can do at home to work with him in sych with what they are doing in school.
if there is individual attention they can give him, one-on-one, or small group, be sure he gets it. summer school can be a great opportunity, he will hopefully be in a smaller class and get more attention, and he will have the continuity and reinforcent right into next year, without the summer break to forget lots of the skills he learned.
and if he reaches the end of this school year and is too far behind, or if he is in summer school and is still not quite ready for first grade, please dont push him ahead if he isnt ready. ask that he repeat kindergarten, and ask for the school to do an evaluation and see if there are any extra services that may help him. please believe me, the earlier the better. everyone freaks out about all the "stigmas" of getting services, but the truth is, its not like it was when we were in school. half of his class will be getting some kind of services, they work it into the schedule (they should anyway, make sure they do) so that they dont miss major new curriculum. and one-on-one or small group services are great, its more individual attention, especially these days with such big class sizes, its actually how education should ideally be for all kids. all kids are different and have different needs and learn in different ways and at different rates. its a shame that many people feel that all kids are, or should be, clones of each other sitting in a classroom with their hands folded. the world isnt like that. any one on one attention you can get him will be so helpful, as long as he enjoys his time with the person. (that is an important point, please keep in mind that learning should be fun for the most part, especially at his age, and his teachers and any service providers of kids this age should know how to do it so it is a fun and positive experience, as should you and your family. this age is so important in that aspect, even more so than the academics, you are pretty much setting the stage for how he will feel about school and reading and learning forever.)
there are many things you can do at home too, the most valuable things you can do will be things that support what he is doing during the day in school, they should be able to give you suggestions. however, if you want some other ideas, if you want to work on skills with him that his class is mostly beyond already, or more fun ideas, ask for them specifically. you can email me and i can give you ways that you can work with him that wont seem like work, which is so important, especially at this age, and especially after he has already put in a full day at school.
best of luck to you, and please remember that the school is there to service your son. you pay for it, he deserves it and has a right to it, and you should get them to give him all that is available and helpful for him. sometimes you have to be the sqeaky wheel. the idea is to find that balance, you are advocating for your son without being a pushy, non-reasonable parent. ideally, you and the school should work as a team to provide the very best education for your son. good luck, D.