You need to ask for a meeting with the manager to work out how best to manage this transition time.
Some settings (more and more as practice develops) ask parents to stay with their child until they are settled.
Others ask the parent to bring the child for the last part of the session so they are not left for long then the time is increased.
Others tell parents to just hand them over and go.
Finally some allow the parent to lead it totally because they are the ones who know the child best.
There are other approaches too. Many practitioners are set on theirs being the one and only 'right' way to do it.
You need to discuss some of the options with the manager and work out what is the best way to work together to make things feel better for your little boy.
Increasing the hours from three sessions a week doesn't sound like a particularly good idea. If he were only attending one I would agree but three should be enough.
Some children do settle and look fine after the parent leaves but it is beginning to be acknowledged that looking fine and being fine are not necessarily the same thing. A parent's gut feeling about whether something is right for their child is probably one of the best ways to make this judgement.
Pre-school can be a good way to get children used to the idea of group care. The ratios are kinder and do allow the staff to offer more support to the children. This means that they can have help to learn to do up their shoes/coat/find gloves/go to the toilet whereas school staff may not have time. It is a stepping stone which probably makes starting school easier.
However that eight months is a long time in your DS's development. My DD started school (after a year in pre-school) at 4.6 but I knew immediately that she wasn't ready and she attended mornings only for the first six months. By that time she was a lot more emotionally ready for the environment and coped far better with full days.
What are you instincts telling you?