C.S.
My Daughter, who is almost four, can spend up to an hour eating a meal. I worried at first, but then spoke to the Doctor about it. He reasurred me that there is nothing wrong with it, and as long as she is eating that is all that matters.
At home I let her be, as long as she is eating and not playing around. Once she starts playing and not eating, dinner or what ever meal it is, she's done. This has worked as she knows that if she does not finish her dinner (or at least a good portion thereof) she will NOT get anything else.
This has worked well for us, and has really cut down the nightly "dinner fight", she also knows that she is NOT to get down until she asks. It took some time and patience on our part, but honest they do get it, and once they do life becomes much easier.
As for sitting with him while he eats, I think she was about 3ish when I began getting up from the table, and letting her sit there. She still had to ask to get down, and knew that if she just played and didn't eat, she wouldn't get anything else..so she sits and eats while I clean up. (of course she is still in eye sight so I can 'remind' her to eat.)
Hope this helps some, and releives some of the anxiety.