How long can my estranged partner have or son (nearly 3 years old) on his own?

Our son will be three in summer, my ex-partner, his father, sees him every six weeks for a week or two. In summer about five weeks. But he is never alone with him for more than three days. My ex-partner lives in England, I live in the Czech Republic. Now, he wants to be with M., our son, four a week or even two weeks on his own. I think that this is not good or M. as he needs his mother around and will miss me. I must say that my ex-partner has a good relationship with M., he talks to him on Skype thrice a week. Can he be longer with our son? Any experience in a similar situation with such a young child? thank you for your advice.