Hi I feel for you and understand, My first son was born severely disabled and is 10 years old he has multiple problems both physical and mental inc severe epilepsy, mental age that has leveled at 2 years old, I also have a 4 year old and a newborn baby and am a single mum, literatually my life is completely given over to my children.
Autism is a differcult decease to get diagnosed but once it is there is a lot of info out there about it, I will also say my niece had a learning delays and instead of talking created her own language that only she understood, very few understanble words about the same age as your son she got the help and within a couple of years she was talking normally now she 12 years old and perfectly ok, in fact is top in her school in maths and has been offered early exams, Sometimes for whatever reason kids are delayed on speech but this might just be a hiccup that just needs to be worked through. My sister used to have fun games trying to make speech fun, lots of rhyming and singing.to encourage her.