EBM via bottle

Hi, have been doing direct latching until now LO 5mths old going IFC... so gotta pump EBM for LO to bring to sch to drink during the day. It seems like LO is rejecting BM in bottle at IFC (teacher said change of clothes due to drool but the bib smells of milk. Tried feeding him a bottle myself and saw him resisting it and got spit out milk onto bib.) There are occasions that he keeps on latching on off and crying, but after he rejected the bottle he was fine doing direct latching... Unlikely to be nipple confusion as we do feed him FM once every night before his bedtime. Could it be that the EBM is too watery or any possible reason he doesn't want BM from bottle? Any sugg how to make milk thicker too? Thanks!