It is possible for him to have squeezed through the bars; the general rule is that the bar spacing should be 1cm or less. Rotastak cages are unsuitable for hamsters anyway as they are too small and have poor ventilation.
For new cages, the Ikea Detolf is a great choice as it's very well priced considering the size of it:
https://www.ikea.com/gb/en/products/...-art-10119206/
You could also try the Kerry:
https://m.zooplus.co.uk/shop/small_p...r_cages/219419
Finally, the Alaska cage is inexpensive and the bar spacing is 0.9cm making it perfectly suitable for even the smallest of Robos:
https://m.zooplus.co.uk/shop/small_p...r_cages/284288
Best of luck
