There is a RESTRICT statement in PROC REG, but it doesn't allow "greater than" restrictions, just equality restrictions.
The problem isn't really SAS. The problem is that when you perform a regression with multiple correlated variables (you have 7 variables in your model), least squares regression (not SAS) can have huge variances for your parameter estimates, and because of that, you can get a negative estimate even when you think it should be positive. In this situation, interpreting the coefficients is a meaningless exercise.
Sometimes using Partial Least Squares regression (PROC PLS) can alleviate this problem, but your R-squared from PLS will not be as good as from PROC REG, and there's no guarantee that PLS will provide positive estimates.