I had this with a rotweiller that we took on from battersea. He had serious possession (and general lunatic) issues so we had to be very careful at first.
We tried everything from scatter feeding (total disaster as the poor thing was so stressed trying to grab every last bit of food) to hand feeding. Swapping from one bowl to another. Adding to his bowl so that every time you approach is bowl he associates you giving rather than taking.
In the end, the only thing that worked was time, we just had to give him enough space to realise that we weren't a threat to his food. We never took anything away from him unless it was a swap. No games of tug o war and treats were ones that he could just eat rather than have as a chew.
I don't want to worry you but i wouldn't be totally surprsed if this behaviour doesn't escalate over the next week or two as he finds his feet. But DON'T WORRY you are doing fine and it will pass. Don't put the dog, or yourself under undue pressure.
We had behaviourists too.
First was a total waste of time and money.
Second was excellent, ex police dog handler, specialising in aggressive dogs. We only had him a few times, but it was enough to give ME the confidence that our dog wasn't inherently aggressive and it was worth persevering with. He had bitten me a few times and with a 50kg rottie i wasn't risking being eaten!
It took about six months i think but one day i was loading the washing machine which is by the back door, the dog had pushed his bowl to the backdoor and there is limited space. My head was inches from his bowl and he was fine, no rushing his food, no gaurding and no eating mumma! That was when i knew it was ok, neither of us thought there was a problem with me being around him when he was eating - six months previous, we had to put the food down and then leave the room before we let him in for his dinner.
It is a very common thing to have collywobbles when you get a new dog, especially a rescue. It'l be grand - well done for giving him a second chance.