Terriers are active and inquisitive dogs - even pet ones ! Some of the working terrier owners will probably disagree, but mine are pets, and I can only tell you what worked for me.
I would suggest more exercise - much more at once, anyway. My small pet terrier x's do at least 2 hours a day, and I don't stroll ! And that's just to burn some excess energy off.
To start off, walk him briskly for at least 45 minutes - get some speed up and don't hang around. I mean walk as if youre going somewhere and youre already late ! Don't let him stop, sniff, pee every 5 yards. Keep moving - you decid