My DS is also on the 4.5 year program and won't commission until next December. Typically his battalion commissions several cadets at that time but he did have to work with his cadre to get it approved. He tells me that at his school it is fairly common for certain majors, particularly STEM, to take 4.5 years. Talk to your cadre and find out what they say.
Another point -- Pima said ROTC is a 1 credit course. I don't know if it varies by school but my DS's MSIV course is 4 credits and requires 4 credits worth of work. And my DS ran out of electives to count the ROTC courses towards by sophomore year. There were only something like 6 pure elective credits in his program. This is one of the reasons he is delayed (change of major is the other) -- he actually has enough credits to graduate now but will be missing a couple of required courses.
Another note on STEM majors -- both of my kids are science majors and find that their lab courses while only providing one credit are every bit as much work (if not more) as their 3 credit lecture courses. They have separate professors, separate homework, separate exams. So carrying 17 credits including a couple of labs is often more like carrying 20 or 21 credits.
While I don't doubt that some cadets are able to maintain outstanding GPA's and that level of credit load but others for various reasons (require more study time, nature of the other course requirements, etc.) may need more time. Talk to your cadre.