Is it safe to take an abx (pivmecillinam) every 6 hours instead of every 8 as it's not possible for me to eat every 8 hours?

Unlikely : I am not familiar with this antibiotic, however antibiotics should be taken at the time frame that they are recommended. The reason for this is the duration of action of that antibiotic; if it's taken out of the timeframe of recommendation for that medication it's pharmacokinetic may not be accurate and hence not have their appropriate affect. Although most medications should be taken with food you do not have to have it with a full meal. He can have it with a small stack and adequate fluid.