Yes the presence of the girls will be having the effect - lots of girls coming into season frequently will cause disruption to boys who are not used to sow pheromones. (Although and as you mention they aren’t neutered - do note that even if they were neutered, the effect on them would still be the same)
If possible, it’s best to avoid bringing sows into a room where boars live at all. (Ideally, the sows being kept in a different room)
If they have to be in the same room, then the sows need to be as far away from the boars as possible (cage dividers higher and wider so scents don’t travel).
If the cages have to be stacked, then the boars need to be at the top and sows at ground level to avoid sow pheromones being able to get to the boys so easily (with the girls at the top then the smells can drift downwards onto the boys).
Always ensure you handle and deal with the boys first; it ensures you don’t risk having sow smells on your hands and clothes and then taking those smells to the boys.
While getting the right cage configuration, and not dealing with the girls first etc can help minimise, it doesn’t guarantee the boys bond won’t be affected by the sows presence in the same room.
You may need to have a shuffle around of cages (making sure the girls are well away from the boys, dividers higher and wider so smells can’t travel/rethink of the girls being in the room).
If you do reconfigure the cages, then do be aware that this can constitute a change in environment for the boys which can cause them an increase in dominance and if there is instability in the bond then it can become a problem. You must make sure you keep the same scent laden bedding in the boys cage - don’t put fresh bedding in. You don’t want to fully clean out their cage at all as removing all their scent can cause a full hierarchy reshuffle and potential problems if their bond has become unstable. This applies to all cage cleans going forward (not just to the time you change the cage set up to get them away from the girls) - only change half their cage at a time.
This guide explains -
Reacting to group or territorial changes: Dominance and group establishment/re-establishment
You will see in section 3 (what do boars need), there is a paragraph part way through that section titled ‘boars and sows in the same room’
A Comprehensive Guide to Guinea Pig Boars
Bonds In Trouble
If Theo is now losing weight then it is because he is not eating enough hay. You haven’t given much information about what is actually happening between the boys but the reduction of hay intake can be because he is stressed by the situation if Alvin is over excited, overly dominant and/or trying to take over the hierarchy.
Bullying and/or the presence of sows has the potential to cause the boys to fall out/fight which would then break their bond.
Switch from the routine weekly weight checks and instead carry out daily weight checks so you can monitor more closely what is happening with Theo’s hay intake.
Theo may need support feeding if he has lost weight and is continuing to do so
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support Levels
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
