1. Do you really need combat?
I am echoing Trish here, but I think this is the heart of the answer and Trish nailed it. While Sorceror certainly supports combat, that is not what its really best at. It is better at the horrors of the things they face in seeking more magical knowledge, in the social interactions where they have powers most around them won't understand and would fear, but far less power than the higher tiered supernaturals. I've never played a pure Sorceror game, but I've had them integrated in mixed CWoD games where whole chapters went by without physical combat (though the threat of it came up much more often).
2. Meat shields are easy to arrange.
If you really want him to have front line support, you don't need the "hey, we'll travel with you to protect you out of pity" angle to get him companions. In the real world today, people travel together for all manner of reasons: companionship, safety in numbers, to have a guide, to share conveyances, etc. This was many times more common and more important in the past.
It would make total sense for him to come across another group who just happened to be going in vaguely the same direction and for them to link up for simple social reasons for at least part of the journey with no other explanation needed. Perhaps some in that group are brawlers that could handle the front lines and perhaps others have no combat ability either that the solo PC could in turn protect. The group dynamics might also give rise to other challenges as well. For instance, depending on his skills he might be well positioned to solve problems they have, including medical issues.
If he is well off, he might also just straight up hire an escort for money.
3. If he expects trouble, he should be able to do something to prepare for problems even alone.
I've largely moved over to Mage: The Awakening and its been a while since I've played with a CWoD Sorcerer, but as I recall a prepared alchemist should be decent in a fight, at least against a being of similar or lower power level. Most higher supernaturals and especially most mages will outclass a sorcerer, but that is by design. An alchemist expecting a fight should be able to prepare potions to make themselves stronger and/or faster, prepare powerful acids or other mixtures that could serve as a makeshift grenade, etc. Even without magic, they likely are carrying substances that would be extremely irritating and could serve as an early version of pepper spray.
In other words, as long as he has a lead time to expect combat and you are giving him challenges commensurate with his ability, he may be just fine in a fight.
4. Meta: Why not just give him experience equal to what the others have?
In some cosmic sense it may be unfair that the other characters earned their current rank and he is handed it, but as a practical matter if you just give him experience to at least put him in the same league as the others you could skip the whole solo travel time completely and move straight to him linking up with the others. This is the fastest way to resolve the issue.