
Just to update on this, and to thank all those who recommended goat's milk yogurt.
My lad has been having a teaspoonful of organic goat's milk yogurt on his dinner every day and is doing very well. There has also been an unexpected but very welcome side effect: for a couple of years his first stool of the day had been (if no action taken) difficult and very painful for him to pass. Veterinary investigation had resulted in no specific cause found, anal glands emptied (without much effect), recommendation to use branny cereal (no easing of the problem), and lactulose, which worked (but with diminishing effect so I was just about to increase the dose). Since he has been on the goat's milk yogurt, his first stool has been firm but is passed without difficulty and lactulose has been discontinued. Cow's milk yogurt (organic) does not have the same beneficial effect for him.
Many thanks, too, to those who recommended Kefir and Saccharomyces Boulardii - having looked into these, I decided not to try them at present, firstly since he is doing so well on the yogurt, and secondly since he has had problems with yeast growths on the past and there is an element of yeast in these products.