Puppy barks and whines when left alone for a few minutes

Hey! I have a 5 month old Maltipoo, that we’ve adopted 1.5 months ago. He’s behaving rather normally for a puppy and sleeps alone at night without major issues. However, we can’t leave the house because he starts barking and whining after about a minute. We’re exiting the house multiple times a day for a few seconds to get him used to the fact that we’re leaving. Also, as our dog trainer adviced, we started going out for a few minutes (5-6). We only come back when he’s been chill for at least 5-10 seconds. He has all sorts of toys, a lick mat, a kong etc., but even with all these distractions he still starts barking and whining. Our dog trainer told us not to extend the training until it gets better, however there has been no progress for at least a week now. Is it too short time to notice a change? Or maybe something is wrong with the training in your opinion? Would you advise some methods that worked for you? We’d like to eventually be able to leave him alone for few hours. I know it’s a long way to go, but that’s our plan. Thank you for all the suggestions.