Ask for the SENCO's report in writing, or ask them to email you and let you know why they consider he has “complex communication needs” because you're likely not to take in everything they say if you ring up, simply because you're so blindsided at the moment.
It will help you to see their concerns written down, then you can go through them one by one and research the meanings.
Does this mean he is on the autistic spectrum?
School cannot diagnose, nor should they suggest a diagnosis, that's up to the professionals who will see your son and give him some fun things to do whilst taking a 'history' from you about his birth, his milestones like what age he spoke, when he sat up, crawled, then walked, did he point at things he wanted you to look at, how does he get your attention, how does he ask you to get him something he can't get himself, reaction to sudden loud noises, soft touch, firm touch, different types of clothing, seams in clothing, is he clumsy, can he follow instructions, what's he like when plans for the day suddenly change, basically just ordinary lifestuff. If you've got a note of these sort of things, or if you want to start noticing these sorts of things from now and making a note of anything that seems excessive or unusual, it can come in handy to give a copy to any of the professionals you'll see.
If he went to nursery, they should have notes of any behavioural and communication observations.
It could be that he can't hear well, so his communication and understanding and speech is behind his peers, that should be one of the first things they test for, before anything like autism is considered.
Please don't worry, right now school have spotted an opportunity to help your son by suggesting these tests, it's extremely rare for any diagnosis to be given in solely one interview, it's usually done over quite a wide timespan involving different professionals like Paediatrician, Speech therapist, Occupational therapist, Educational Psychologist (usually called Paed, SLT, OT and Ed Psych) who will meet you and your son then they all get together and decide what if any problems exist and what they can do to help.
SNChat and SNChildren are busier boards than this one