I totally understand your concerns, I would be worried about this situation too. As a mother, I always vow to go by my instincts....I have been ultra persistent with my doctor when i've thought my daughter was ill because i'd rather follow my instinct and be wrong than not follow it and wish i had!
Is the pre-school on holiday now or is he attending through the holidays. I genuinely would try a different pre-school (if possible, obviously).
Of course as parents we are biased, but were supposed to be, aren't we. If you're not happy that everything is okay, you should at least arrange a meeting to discuss further and fully.
My daughter started pre-school aged 3, last year and thankfully settled in great, but i found her starting stressful enough anyway (more for me than her), that if something was wrong, i'd tried to sort it out and still i wasn't happy, i think i'd pull her out - as i dont think i could face leaving her there. whilst i am trying to encourage her to have independance and confidence, she is still my baby.
Good luck with this situation, x