Lactose free or soy based milk

hi mummies! i need your kindnopinions on this matter. My son is 5 months old and is having diarrhoea for the past 3 weeks for every 2-3 days once. Only recently the diarrhoea is 5 days straight. He also has rashes on his face Doctor suspect he is lactose intolerant and recommends i Change his milk to lactose free or soy based milk. Baby is currently drinking S26 Gold. Any recommendations on which milk to switch to? I am very scared that he will reject the new milk as I only have enough fund to buy 1 milk#pleasehelp