My son is 34 months old and has delayed speech. He is seeing a speech therapist but my biggest concern is sometime he doesn't seem to understand what we are saying. He will do things when asked, like wash his hands, brush his teeth etc. but doesn't seem to understand things like daddy is going bye bye and will be back later. Seems like a almost 3yr old should understand that daddy has to leave town and will come back. Can someone please help me and give me some ideas/suggestions?

Get his hearing checked by an ent.

Make an appointment with your health visitor, there might not be anything wrong though but trust your instincts. If there is anything wrong, he's probably still too young to be diagnosed. My son was diagnosed with Aspergers Syndrome when he was 5. don't worry, you're a good mum xx

Children that age do not understand object permanence. Just like a dog, I hate to use that analogy, but it holds true... once someone leaves they think that they are gone forever. Object permanence takes time for youngsters to comprehend. Some understand earlier than others, but he is still not out of range for his age at this point.