Jim Duffy's side came from behind to seal a deserved victory which was engineered by the cultured left foot of former Motherwell and Morton midfielder Forbes.
The 32-year-old forced the first save with a deflected effort but the hosts took the lead in some style a minute before the end of a tight first half.
Left-back Robbie McIntyre collected the ball after a good spell of passing and powered an angled drive into the far corner.
Forbes had an effort parried just after the break and the midfielder's quality on the ball told in the 53rd minute.
He got the ball back after a short corner and whipped in a cross that only needed the slightest of touches from Ryan McGeever to see it into the corner of the net.
Forbes broke forward in the 67th minute and lashed a swerving shot that Calum Antell could only palm into the path of Tomas Brindley to stab home.
Brindley was wrongly booked for diving before forcing a decent stop from Antell and the Sons had their third when Neill rose well to power home a header from Forbes' free-kick eight minutes from time.