How are you? Reading your post I can imagine how alone and confused you feel inside, extremely isolating by the sounds of it. Let's think this through..
How's his physical health, to your knowledge? Any indication of worry or little changes you've noticed? Going to the loo more, sleeping more, could be anything. Sometimes men will subconsciously lock themselves off when dealing with health issues, would he share his concerns with you in that regard? If not please put your arms around him and gently ask. If he's fine healthwise but you're just aware something's off, perhaps there are other problems on his mind, matters financial for instance? Again, same routine. Often, a very subtle act of affection can really open a man up to talk. Try to mark off these possibilities before considering a potential 3rd party being the cause. Are you satisfied you've resolved this previous issue with his work colleague? Because if he's being weird around his phone use in your presence (turning it to face away from you when texting, not taking or making calls in your presence, paranoid or jumpy when notifications pop up etc) it could possibly indicate something else amiss.
Ask for him to show you his messages or whatever else you want to see. Don't sneak or snoop, just gently ask. There is NOTHING wrong with asking. If not you who in the world else has the right? It's given you anxiety in the past so don't be afraid of asking. His reaction will of course be key.
And don't worry about making him feel "fed up as well" if it's his behaviour causing you to. Be bold in asking what you don't know the answers to, just not accusatory. He's your husband of 28 years standing remember? He wouldn't be if he didn't love you back. I personally don't think he's upto any sort of skulduggery, but it's upto you to be certain of that.
Look after yourself