First of all, in the world of cat socialisation, 2 months s a very short mount of time indeed. For your 9 year old cat who has been used to having his own territory, a new cat turns his whole world upside down and is a massive change to get used to. It's vital that we take things really slow and not force their relationship.
The truth is that most cats, if given the choice, would prefer to be lone cats. They don't typically enjoying sharing territory so we need to be respectful of this. They may never be best friends but we can certainly aim for them to co-exist without stressing each other out.
While I do understand the desire to introduce them quickly, this process needs to be a very slow one (weeks to months). While they have already met, they are not yet accepting of each other so we should try to re-introduce them at a slower pace.
Separating them at this point is a good idea, and they may need to be separated for some time longer.
First things first, prepare their territories if not already done. Freddy should have a designated area that is his and his alone; an area that has their own food bowl / water bowl / bed / hiding place / toys / litter tray/cat tree etc. We cannot expect the cats to share ANY of their things at this stage, as this will likely only lead to conflict. Similarly, your established cat should have an area that the new cat is not allowed to go in to. When they understand their place in their home, they are less likely to invade each other's territory, creating jealousy and leading to chasing and other negative behaviours.
After a few days, place your new cat in a new area of the house (with all the amenities it needs) and allow the settled cat to enter the new cat's old territory. They will be able to smell them and prepare for their meeting. Wait a few days while your settled cat gets used to the new smell and the idea of a new arrival.
You may also choose to mix their bedding at this stage, so they each sleep on the other cat's old bed.
The next step is to allow them to smell each other through a closed door. If they both tolerate this well, the next day consider allowing them to 'meet' through some form of mesh/cat crate, so they are safe but can 'touch' and smell faces if they like.
If at any stage either of the cats show negative behaviours e.g. chasing/ hissing, take them back to the previous day's exercise. During their 'sniff sessions' it's great if they can each have someone with them who is stroking them and offering them treats so they understand it is all a positive experience.
I appreciate this may seem excessive as they have already been living together of a couple of months but it can hepl create a more accepting and understanding relationship.
Once you (and they!) feel ready for a real meeting, give them both access to a neutral room that neither of them sleep in or have belongings in. It's best to not have either cat picked up in your arms so they have the freedom to do what they choose. Ensure they each have an 'escape route', as well as high pieces of furniture or cat trees to run to. Quite commonly at this stage, once cat may choose to run away and hide. This is fine; let them take things at their pace & do not force a meeting.
The first few 'meetings' may only last a minute or two. This is fine, either cat may be finding things a little overwhelming.
However, you may find that they take to the process really well and you can take things quicker than most thanks to them knowing each other to some degree.
If you haven't already, invest in a plug-in adaptor called 'Feliway Friends' which can be plugged in the wall and will release 'Happy cat pheromones' to encourage bonding and a more settled friendship.
Consider giving wither cat a few weeks of calming supplements in their food to take the edge off. These are safe, all natural products that can really help. Examples of brands include Zylkene, Yucalm and Nutracalm, though there are lots out there. You can get them from vets, pet stores or online.
Ensure both cats are having their mental needs met with plenty of toys, interactive games, food puzzles etc.
With the older cat, if not done in the last few months it may also be worth having a vet check. At their age, we can start to see early arthritis, dental disease etc. which may make them a little grumpier and less willing to accept Freddy.
With regards ***** ***** flap, you are likely to find that as they grow to accept each other, this behaviour will change. If you see the older cat guarding the flap, take their attention to something else in a different room as this behaviour is not acceptable and is probably quite stressful for poor Freddy.