How stressful for you
The way you describe him sounds *exactly* like my friends now 3 year old, let me tell you about him and perhaps it can help put your mind at rest a little.
He was like Milo (still kinda is actually) when he was little. He was obsessed .. I mean OBSESSED with putting things in a row, cars, blocks, pens .. anything, they where in size/colour order and such. He was never really interested in traditional little kid toys. Then he became interested in electronics. He could go on her Iphone (at 2) download apps and play them from the get go. At 2 he could count to 100 forward, backwards, in 10s, knew his alphabet, his colours .. very quirky little boy.
Although he knew/knows his numbers his speech has never really developed. He only says single words and only says them when asked, he does not initiate conversation, he couldn't ask "mummy I need a drink" or tell me "Hi Chelle" when I walk inn, like most 3 year olds. Even when he does say his words, he does no pronounce the whole word.
He freaks out at the most odd things, he can't stand loud noises, things like the vacuum, people singing happy birthday, he totally freaks out and so on. He won't eat, he only eats certain textured foods like pretzels. He hates to be touched. He just screams all the time (has since a baby) and is just a general gump ass lol.
His parents have been worried about autism .. I mean if you read the above could you blame them? Also about his hearing because of his lack of pronunciation of the actual word (His sister has hearing loss in her left ear also)
She took him to his 3 year check. She said regarding his ... 'quirks?' it's just him being him. She checked him over and watched him interact and was happy.
She said he's very smart for his age with his numbers and such. With his speech, she agrees he should be saying 2 or 3 word sentences at this point, but she said, a lot of the time when they are a bit delayed in speech it's because they have no need to really speak or ask for things because for example if they are at home all day with mum, she learns to pre-empt his needs so he never really needs to ask. Also in his case, if he wants .. for example a paper towel, he will get a chair, take it to the kitchen, climb up and get it him self yanno? If he wants a bit of cheese, he'll open the fridge and get him self a bit of cheese.
She also said that girls come out the womb talking and being interactive with other kids, little boys tend to just play quietly next to other kids and that's enough company for them
She is sending him for speech therapy and a hearing test .. she also suggested a couple days of pre-school a week, which is mother is having a nervous breakdown over haha. The ped seemed to think that another figure of authority and a situation where he has to ASK for what he wants/needs will bring out his confidence and speaking abilities.
I think that .. more so in this day there seems to be this unwritten rule of how kids are supposed to ack by a certain age and what is 'normal' (what ever that is!!!) or they have a problem, it kinda drives me nuts because it worries people unnecessary ..
However, with that being said, trust your mummy instinct .. it's there for a reason, you know Milo better than anyone, you spend the most time with him. Our guts are not often wrong. Every time Autumn had an ear infection, I just knew in my gut before we even got in the Drs door. Go speak to your Dr .. Ask to get referee to someone and have him checked out ... what can it hurt right? And if nothing else it will give you peace of mind.
we've been noticing little signs for awhile now. the frst one was his love to always stack everything since he was about 10 or 11 months. today the conversation came up becaue we were talking about how Ava smiles already and she's only 5 weeks and it seems early. milo didnt start smiling until about 2.5 months. and i said how milo has never really smiled at anyone. if you smile at him he has never smiled back. the only time he's ever smiled was if you tickle him or he thinks something is funny. but he's never smiled for the camera as he got older. he just always seems like a super grumpy baby all the time.
then his lack of speech makes us worried too
i've looked up videos that showed examples of how children with autism act (like signs to look for) and he seemed to have the majority of the signs. but a lot of the "signs" look like normal kid behavior.
Well done for keeping calm with the rash - I'm not very good with things like that and tend to fear the worst. 
Still, it's only pre-school and to begin with, if I get into one of the 2 I want for him, he'll only be going for 2x2hour sessions per week. 
though i managed a little nap with ava on the couch for about an hour and a half. i should be going to bed since OH is up with the kids. 