Constipated dog, should I be concerned?

Hi there, my 7 month old yorkie-apso recently went to the vet due to repeatedly being sick. Was told it was due to us changing his food and to stick to plain foods (chicken and rice) for a couple of days. Since that visit he appears to be constipated, keeps getting into position but doesn’t seem to be able to defecate. It’s been about two days now and we are starting him back on his normal food (the food before we changed it initially) which I’m assuming/hoping will help him as it’ll have more nutrients and fibre he needs but wondering if I need to be worried and if there is anything else I can do to help?

Is this definitely constipation?  It is worth bearing in mind that some constipated dogs are actually wiping their anal glands or trying to urinate, or that they are struggling with back or abdominal pain (e.g. they strain but then stop straining cos it hurts).  Being unable to pee is obviously an emergency.  It sounds as though, two days later, the bowels literally haven't moved on much - is there faeces there, or were the guts emptied as a result of the vomiting?  Might they be dehydrated? - dry faeces can be harder to pass.  Can you rule out blockages or a hernia?  In cases such as these, allowing a dog to continue to strain can be damaging.  I would recommend contacting the vet so that they can take a proper history and assess how soon an appointment is needed.