In my experience, only your horse can answer that question. Assuming you have had an insulin test showing his insulin level is currently low, get to know your horse’s baselines indicators now before you start him on grass:
Can you find a pulse on any of his feet. If so, which one and what does it feel like. If there is any change on the number of feet or strength of pulse, back in dry lot and start over.
What does his crest look and feel like now? Is it straight. Is it thin from side to side? Is it loose and supple if you shake it back and forth? It it gets more arched, thicker from side to side, or firmer when you try to shake it, back in the dry lot and start over.
If you miss those first two signs, you will next notice ouchiness over gravel (if barefoot), then a fatty lump in front of, well it forms in front of my mare’s udder. I think it was in front of my gelding’s penis not behind, I don’t have a clear memory. Then an overall weight gain and the well-known lump over the tail. All of these mean hightail it back to the dry lot and stay there until all symptoms return to normal and/or get an insulin test before starting again.
Have you joined the ECIR IO Group? Great people sharing a wealth of information on IR.