Concerned about child possibly within the spectrum

I'm no doctor but I feel that my fiancé's four year old son may be in the autism spectrum. He is not toilet trained yet and resists even giving it a try. He just screams and runs off. Actually he screams all the time about anything that frustrates him. He doesn't have communication skills yet, he can say complete sentences but he doesn't respond to questions, doesn't understand the difference between yes or no and repeats the same phrases over and over. He is the same with activities, he repeats the same thing over and over and this goes on for months until suddenly one day there is a new obsession and the old one is forgotten as if it never existed. He bites and hits his Dad. Usually when he's in a situation which creates anxiety for him. Such as meeting a new person. They might say 'hi little guy how are you?' And he will not know how to act or respond so he will hit his Dad or throw things. He also puts his hands over his ears. He also throws things around at other times for seemingly no reason. Such as going to his bookshelf and throwing all of the books onto the ground. He doesn't play with toys the way other children do such as take his cars around the race track he finds it more fun to throw them in the dogs water bowl and look at them laying on the bottom. He doesn't colour or look at books. If you try to show him a book he grabs it and tears the pages out. He will watch someone else paint or colour but when you try to get him to join in he will scream and chuck all the crayons into the air. He loves other children, however some don't understand him and think he's strange. When we had four children visitors who were aged 1,7,9 and 11, he chose to crawl around on the ground with the one year old while the other kids played on the swings and trampoline. He won't play on his swing set at all but he loves to go on the slide if we take him to the park, just not at home. Same with bikes. He will sit on them but refuses to try to use the pedals or hand bars, I think he feels a bit frightened and only wants to be pushed around or towed. There are other behaviours occurring also that present a high possibility of special needs. This is ruining my relationship though as I am finding it painful to see all this and no help being seeked, but my partner says he's just four he will grow out of it. And he won't get a referral for help as he feels having only 45% of the custody, that he doesn't have the right to seek help behind the mothers back and when he asked her about it she got very defensive and said there's nothing wrong with him. I really want to get help for us all as a family but my partner isn't budging. This has caused lots of arguments or a year now and I have just recently moved out as my final desperate attempt in making his Dad see that this is serious. I know this is not my boy, but he stays with us for half of every week and it is greatly affecting the entire household. Waking our baby up with the screaming and having to keep her away from him incase he throws things. So in order to save our family I am hoping that if I can find the right avenue to begin helping his son, he will come on board. But I don't know where to start and I'm scared I'm wrong and feel like I'm picking on the child. But the more research I do the more I feel that there's problems here that will not just go away and we need help. Does anyone have an autistic child? Do the behaviours of this child sound anything alike? Also does anyone know of a good starting point for me and my partner to get professional help?