Eight weeks or so isn't an especially long time, so he won't be hugely unfit. As your main aim is dressage comps, your work needs to be tailored towards conditioning his joints to working predominantly on surfaces and grass. His tendon fibre turnover will need a kick start - more so if his turnout has been restricted because of the weather - and the osteocytes will need to set to work remodelling his bones and making them more dense (and hence less likely to throw a splint or a fracture). For me, the most important part of a fitenning programme is the initial phase - this is where you lay down the foundations for a healthy horse; as I mentioned previously it is important not to to overwork him at this early stage or you risk pulled/strained tendons and ligaments, and the risk of throwing a splint is increased.
I would start with a week of walking - start off with 20 minutes and build up to 45 - 60 minutes by the end of the week. If you can, hack out as the mild concussion of walking on roads is excellent for starting the remodelling process and "hardening" his legs. If you're confined to a school or field, setting up poles and cones will add variety for him (and you!) but don't expect wonders of him - his muscles are soft. If he was working in an outline before his rest, then still expect him to do so. It obviously will not be as established as before as his muscles have softened, but he can still work on a basic contact going straight and fairly round. Tootling along on the buckle end will only mean he takes longer to get fit (obviously give him a stretch out at the end, but generally work him "properly").
In week two I would introduce trot. Again, if out on a hack do it there. If not, then practice in the school but bear in mind his balance won't be as great as it was before so stick to big circles etc. Regular short bursts interspresed with periods of walk will be better for him than one long trot. Towards the end of week two he should be comfortable trotting for around 10 minutes before needing a rest. In this stage you want him to work little - so his heart rate should increase and he may become "warm" but he shouldn't be sweating excessively. As your aim is dressage, I would be introducing 20m circles, figures of eights etc in the school/field, again asking him to work correctly into his outline.
In week three you can introduce canter work. As with the trot, do it little and often. In trot you can re-introduce basic lateral work (if he's capable) and 15m circles, serpetines etc. By now his outline should be coming up to the same level as before his rest.
In week four you're "up and running" as it were, and he should now be fit enough to school for 45 minutes without needing resuss at the end!
This is just a basic outline based on what you've said - it will vary depending on your horse's age, schooling level prior to his rest and also his routine over his rest (if his turnout has been restricted, I would drag the four weeks detailed above out over six). If he's a mature horse, he will have "muscle memory" and should be back up to fitness soon. However if he's younger then it will take longer.
To vary day to day riding, use cones and poles (you can raise them once he is fit enough) etc. Setting out a line of trotting poles and raising alternate ends will be a good exercise to help your dressage as it will regulate his paces and increase his elevation. Remember to practice lots and lots of upwards and downwards transitions - simple at first and then building up to more complex ones as he becomes more supple and fit. Introduce lateral work to lighten his forehand further and encourage him to come up and forwards into your hand. Try riding centre lines and quarter markers too - don't get stuck to the wall of the school. This will help you as you have to ride the outside of the horse as well as the inside, which will lay the foundations for higher level movements when you and he get there.
Hope this helps
