His bond being only $15,000 is definitely a good sign, since it means that the police are actually (seemingly) keeping their word, since normally bail on a felony 2 would be around $25,000 at least.
Being on probation for the joint is really not a worry, he won't receive any additional time for that, since the the felony 2 is a much bigger deal. Technically, he could get 3 to 10 years for 2nd degree burglary, but due to his lack of criminal history, his age, and the police actually rewarding him for his honesty, I would think he'd get 3 at most, which could be 1 1/2.
Unless you think he has a chance at innocence too, it does make sense to keep him locked up to reflect on his misdeeds, and just because if he's going to have to go to jail anyway, he's getting credits against that time, so it's less time he'd have to spend in the future.
Hopefully though with his situation, the D.A. or judge could use his/her discretion to give him 4 years of probation as an alternative to jail.
Under Illinois law, Burglary is considered a Class 2 crime which is punishable from 3 to 7 years in jail. However, as an alternative to jail, up to 4 years of probation may be available if the defendant has not been convicted of a class 2 felony or greater within the previous 10 years.
Since your son hasn't been convicted of a felony ever (let alone the last ten years, he should hopefully be granted a light county sentence, maybe even credit for time served--and the probation, which should hopefully keep him on the straight and narrow from now on).