Hello from the future!
He does not doubt that there are: — Fooloso4
But then in later Meditations he does indeed, so then he gives arguments for the existence of the outside world (not of material things yet). One of the arguments is the origins of his ideas, which is also one of his arguments for God.
But how do I know that He has not brought it to pass that there is no earth, no heaven, no extended body, no magnitude, no place[...]
It is only then later that he tries to prove whether there are res extensae.
In the third meditation we see an argument that resembles what is said in the quoted paragraphy from the 1st Med., that if I don't see the objective reality of an idea anywhere (it is not me) in me, either formally (it exists as a thing, like thoughts) or eminently (it is less perfect than me, so it can extrapolated from), the outside world must exist so that there is something to cause that idea.
However, just one/two pages later, he throws that out:
For [even] when I think that a stone is a substance, or at least a thing capable of existing of itself, and that I am a substance also, although I conceive that I am a thing that thinks and not one that is extended, and that the stone on the other hand is an extended thing which does not think, and that thus there is a notable difference between the two conceptions—they seem, nevertheless, to agree in this, that both represent substances. In the same way, when I perceive that I now exist and further recollect that I have in former times existed, and when I remember that I have various thoughts of which I can recognize the number, I acquire ideas of duration and number which I can afterwards transfer to any object that I please. But as to all the other qualities of which the ideas of corporeal things are composed, to wit, extension, figure, situation and motion, it is true that they are not formally in me, since I am only a thing that thinks; but because they are merely certain modes of substance (and so to speak the way in which corporeal substance appears to us) and because I myself am also a substance, it would seem that they might be contained in me eminently.
And:
Hence there remains only the idea of God
I personally don't fully agree with the argument above; not with the conclusion, but with the way it is presented. Res extensa and res cogitans have different attributes, and so have different modes. It is a possibility that a res cogitans is
born ex nihilo with the idea of extension (and in which case that idea of extension would not represent anything real); but without being so, it could not develop it by itself, as it does not have extension in itself. For the ideas to be contained in me, they would have to be simply a fortuitous result of the operations of my mind as it was born, in which case they are contained formally, not eminently.
It is also through God that Descartes "proves" in the 6th Med. that there is an outside world. But it has several problems.
On one kind of interpretation, Descartes relaxes his epistemic standards in the Sixth Meditation. He no longer insists on perfect knowledge, now settling for probabilistic arguments. — SEP's Descartes' Epistemology
So perhaps Descartes is never fully convinced that there is an outside world — I have some more reading to do to see if he says otherwise somewhere else. But he is not a skeptic, he does not need to be, he only finds many strong arguments for why there is an outside world.
As a curiosity, the bit on wax on the 2nd Med. seems to be somewhat of a proof of the outside world:
Is it not that I imagine that this piece of wax being round is capable of becoming square and of passing from a square to a triangular figure? No, certainly it is not that, since I imagine it admits of an infinitude of similar changes, and I nevertheless do not know how to compass the infinitude by my imagination, and consequently this conception which I have of the wax is not brought about by the faculty of imagination.
A very muddy, but wonderful book.