You are partially correct that it is generally best to wait to do cognitive or academic assessment until at least 1st or 2nd grade because testing earlier is not very reliable. But if the school is suggesting the need for additional assessment at this time it is probably because they suspect that your son may be in need of additional support which is not available through his speech services. Some developmental disabilities such as autism or cognitive impairment can be identified at a very young age, and early intervention is extremely important. I would trust the school's guidance.