slipped in a way that his urethra and foreskin exposed and came into contact with you (which looks highly unlikely), yes then you are at a risk and need a test after 6-8 weeks, followed by a conclusive one after 3 months.
If during the event, it only slipped a bit on his shaft, then there is no risk, For status of someone, verbal assurance is not enough when the risk of yoru won life is involved.
if it slipped inside anus than there's risk,if slipped during pulling out theres not measurable risk... and rate of hiv by slipping is far more less than breakage..
u may get tested at about 6 weeks.. it might be conclusive for you..