My Boxers character has suddenly changed

Question
I have had Bruno my red male Boxer since he was 3 months old. He has been
through obedience training around that age, and is extensively socialised with
both humans and other dogs in the neighbourhood. He has been an absolute
angel, aside from a few naughty bouts of chewing the furniture and plants,
but nothing more. He would come on command, sit, stay for extended
periods impeccably.
Unfortunately, all these changed the week of his 3rd birthday. He no longer
follows me around the house wherever i go, he would not even bother to lift
his head when i call him to come. He seems suspicious and weary one
moment, aloof another, and yet playful as usual the next. Once, when the
gate to the yard was open, he even ran away, though he did turn back
periodically to see if i was following. Whenever i got near, he would leap
away. He never used to behave this way.  I no longer trust him off leash.
I cannot explaimn the sudden change. I feel most heart broken that the
relationship is different now, and i try to spend more quality time with him,
but so far it has not helped.
I hope you can shed some light to help me adjust.  

Answer
Hello KC,

Are you in the UK? I ask because of your spelling :)

Bruno is now 3 years old, correct?

A personality and attitude change, especially a sudden one, is usually do to something in the family dynamics changing.

Have you moved recently? Had a baby? Bought another pet? Gotten married or divorced? A new boyfriend or girlfriend? Any change whatsoever?

When something changes in the family, it is a change in Bruno's environment. This can cause behavioral changes.

Search your mind for any type of change that might have caused this, from when you think he changed.

Spend more time with him, play more often, and do not allow him to be off leash unless in a fenced in yard with the gate shut of course. This is for his welfare. He might smell a female in heat and go for it, and he will not obey you under those circumstances.

Do everything you can to reinforce the family dynamics and routine. Any routine change can bring about a personality change.

Stick to play times, walk times, treat times, and the like.

If he doesn't show an improvement, take him to your vet. He might have an illness that is causing him discomfort and the side effect being a visual change.